Part 107 UAS pilot reported a near miss with a low flying helicopter operating at a nearby medical center. Neither pilot took evasive action.

Narrative

On Day 0; I conducted two autonomous flights to capture imagery of a hotel located in Class G airspace. Two flights were planned; the first approximately 2 hours before sunset (XA:33 Hours); utilizing a DJI Mavic 3 Enterprise. An autonomous flight plan was created; capturing RGB images along parallel flight lines oriented 258/78 degrees with a 90% overlap directly over the hotel property at approximately 165' AGL altitude. Upon completion of the autonomous portion of the flight; oblique images were captured by manual flight around the property's perimeter. The UAS captured images with the camera angled at 45 degrees and oriented towards the building throughout the flight while manually flying around the entire perimeter at 5-7 MPH. Upon completing the perimeter flight/oblique image capture; the UAS pilot decided to execute a second autonomous flight; adjusting the UAS speed to 9 MPH vs 15 MPH of the first autonomous flight. All three of these flights (autonomous flights 1 & 2; plus 1 perimeter oblique flight) were conducted without incident and concluded approximately 1.5 hours before sunset. The second half of the mission was flown with a DJI M30T; a UAS with a thermal sensor to capture IR images of the hotel's roof. This flight could not begin until 30 minutes after sunset (XB:03). While waiting for the flight window to open; I heard a helicopter close by and noticed it approximately 1/4 SM East; likely landing at the medical center approaching from the south. Coincidentally; I took a photo of the helicopter at XA:48; just before it disappeared behind the buildings. As soon as the thermal window opened; the M30T was launched and commenced its autonomous flight. The UAS was operating at 188' AGL; and the flight was expected to be completed in 7 minutes. The flight path maintained the orientation of 78/258 degrees and a 90% overlap. About midway through the flight; the helicopter was heard; but it did not sound like it was taking off; or if it was; it was hovering for a period of time. The helicopter suddenly appeared; the drone was on a 78-degree leg and directly over the hotel. The helicopter was on a westerly flight path at a very low altitude and high rate of speed. I noticed dual landing lights on the helicopter's nose were illuminated; which assisted in assessing the altitude and direction of flight. The instant I determined the helicopter's flight direction could conflict with my UAS operation; I anticipated terminating the flight and taking evasive action. The UAS anti-collision beacons (strobes; upper and lower) had been activated before launch; and the helicopter pilot likely saw them. While they did not appear to take evasive action; the helicopter passed just south of our operation; parallel to the UAS's autonomous flight path. The altitude at which the helicopter pilot departed the area was too low. As a manned pilot with several hours of helicopter flight instruction; I understand helicopters operate at much lower altitudes than fixed-wing aircraft. Remember that the UAS was operating at 188' AGL - but only 90-95' above the hotel's roof. As a pilot since 19XX; with ASEL/AMEL ratings; and a CFII for 30+ years; this is the first time I've filed a report; and the first time in nearly 3;000 hours of flight time it felt necessary; not because this is sometimes viewed as a way to cover yourself. I'm not concerned about the compliance of the UAS flight - but more concerned that the helicopter flight could have put themselves or someone on the ground in danger if their flight path had been a few degrees to the right.