Government helicopter pilot reported an NMAC with a helicopter from another public agency during a mission. Evasive action was taken to avoid a collision.

Narrative

I am documenting this incident due to the NMAC incident and to place it in record to assist safety in the future.Public Agency X is a public use police/fire/rescue governmental agency operating two helicopters. Both helicopters have a nationally registered call sign.Incident SummaryOn Day 0; at XA45 hours; Aircraft X monitored a dispatch to our jurisdictional parklands for lost hikers. The call subsequently escalated; with one hiker reportedly experiencing a medical issue. In accordance with our mission responsibilities and operational policies; Aircraft X responded from ZZZ Airport. I was the pilot in command; with Person A as crew.At XA55 hours (dispatch timestamp); we advised dispatch that we were nearing the scene and I requested that ground units hold while we assessed the feasibility of landing for direct extraction.We arrived on scene and initiated a left-hand orbit to locate the subjects using the GPS coordinates provided. Despite not receiving a clothing description; we identified individuals under a tree on the trail and continued a left orbit to conduct a reconnaissance for a potential landing zone.Moments later; Public Agency Y helicopter (Aircraft Y) unexpectedly passed us in the opposite direction; slightly lower than our altitude; at an estimated distance of approximately 150 feet off our left side. This event qualifies as a Near Mid-Air Collision (NMAC). I was startled and confused; as we had not heard any radio traffic from Aircraft Y on any common frequency; nor were we aware of any coordination regarding their response to this call.I immediately reduced airspeed; climbed and maneuvered behind Aircraft Y to maintain visual contact and avoid conflict. I began attempting contact on the helicopter air-to-air frequency.Air-to-Air Communications ContextThe Aeronautical Information Manual (AIM)--a non-regulatory FAA publication outlining aviation procedures and best practices--specifies 123.025 MHz as the helicopter air-to-air frequency. Historically; Aircraft X monitored this frequency (COM2); but we consistently experienced difficulty reaching other law enforcement air units in the region.After attending regional safety fly-ins and consulting with peer agencies; we learned that most air units in the area utilize XXX.XXX MHz for air-to-air coordination. This appears to stem from fixed-wing pilot origins within these agencies; where XXX.XXX MHz is commonly used.Despite our prior efforts to encourage agencies rotor assets to utilize 123.025 MHz; we ultimately adjusted our operations and began monitoring XXX.XXX MHz to maximize situational awareness and deconfliction with commonly shared airspace users. (Public Agency Z remains an outlier; using XXY.XX MHz.)When I failed to receive a response on XXX.XXX MHz; I attempted contact on XXY.XX MHz; the Common Traffic Advisory Frequency (CTAF). I received no response there either. Aircraft Y began transmitting on a Police Channel; instructing us to switch to 'helicopter air-to-air' to avoid a 'midair' collision.Although I had maintained visual contact with Aircraft Y since their unexpected sighting; I agree that positive radio communication was essential. Upon hearing their reference to 'helicopter' air-to-air; I promptly switched to 123.025 MHz. While Aircraft Y was technically correct in using the designated helicopter frequency; our unit's modified practice of monitoring XXX.XXX MHz resulted in a failure to establish timely communication.This presents an operational dilemma: reverting exclusively to 123.025 MHz may result in further missed communication with other regional operators who we fly with much more often. However this practice will be reviewed.I was surprised by the possessive claim of airspace; which is not applicable in Class G (uncontrolled) airspace. Additionally the rescue call was well within Jurisdiction X; not jurisdiction Y. Initially; I thought he was referencing controlled airspace near ZZZ1 Airport. At that moment; both my Tactical Flight Officer (TFO) and I observed what appeared to be an aggressive maneuver followed by an abrupt landing on a nearby hill. It appeared to be a rushed attempt to claim the rescue scene. We elected to disengage from the scene and departed the area.Person B on scene later contacted me and confirmed that Public Agency A personnel on scene had not requested Aircraft Y; and were unaware of their deployment.Immediate ActionsI immediately met with our Operations Captain and briefed him on the incident and my concerns. I also preserved the following data:-Cockpit video and audio recordings of the radio traffic.-Flight track data from our onboard mission computer.-Flight paths showing Aircraft Y's arrival at 1;500' MSL and Aircraft X's arrival 30 seconds later. Initial Observations-Both helicopters arrived in a contemporaneous time period and were likely focused on ground search; diminishing their active see and avoid scanning.-At the time of Aircraft X's arrival over the area; Aircraft Y was on an extended downwind placing them behind and below Aircraft X's path making visual identification difficult.-Neither aircraft was initially aware of the other's presence due to a lack of interagency communication. This likely resulted from mutual assumptions.-If Aircraft Y was dispatched by Public Agency A; they may have mistakenly assumed no need to coordinate with Public Agency X. However it is apparent they just overheard the dispatch and acted autonomously.-Aircraft X responded under the assumption this was a Jurisdiction X-only call. We had no information suggesting Public Agency A's involvement.In contrast; another pubic agency routinely coordinates when conducting rescues in our jurisdiction--either through direct radio contact or pre-departure notification. When Aircraft X is unavailable or when Advanced Life Support (ALS) capability is needed; we gladly welcome their assistance.Preliminary Conclusions-This incident has highlighted opportunities to improve interagency coordination and reduce assumptions in rapidly evolving rescue operations.-I accept full responsibility for our part in the delayed visual scanning and will reinforce this in the future.-Moving forward; we recommend establishing standard protocols for interagency aerial response; including required communication prior to scene arrival.Public Agency X Mitigating Actions Moving Forward (Pending unit safety stand down; meeting and briefing. Current policy review should be undertaken and modified to include best practices.)When conducting any operations outside of jurisdiction X; continue our standard practice of announcing our presence on allied agencies primary dispatch channel and determining if they would like our assistance.It is suggested that when responding to search/rescue/fire reports; even within our jurisdiction; we now take extra efforts to enquire and investigate if another agency has involved themselves.Public Agency X dispatch personnel should make elevated efforts to pass on to Aircraft X any knowledge of other agencies' involvement.Suggested discontinuation of routine use of ADSB/Mode S off operation.-Although it is the recommendation of the FAA coordinators to just leave this feature enabled; this incident clearly demonstrates a potential traffic alert safety concern. Secondly; the technology presents certain ADSB traffic notice deficiencies. Going forward it is suggested to only enable this feature when engaged in specific law enforcement activities in which it would benefit bad actors to track law enforcement aircraft.Reinforce; through routine briefing; training and practice; see and avoid active scanning; emphasizing pilot heads up during active incidents. ('Where there is none; there is one. Where there is one; there is many.')Examine potential for communication monitoring to be improved on multiple air to air frequencies. -Additionally; recommend to chain of command the exploration of a regionalair support universal channel on Public Agency X. This would allow continual; more reliable; clearer communication amongst all regional air assets; and further give agency dispatch centers immediate and uninterrupted access to immediately inquire if there was an air resource in position to help with quickly developing situations.