Government UAS pilot reported departing their assigned altitude without a clearance.

Narrative

At approximately XA:00 on Day 0 Aircraft X was performing a routine patrol over the lake at FL210 in conjunction with the Public Official Agency. The pilot was flying while monitoring an ADS-B display adjacent to the Pilot In Command (PIC) station. At approximately XA:03 Aircraft X received a call for aerial support for a search for a missing person. At this time the pilot was communicating with the ground units to move the aircraft into position to effectuate the search. The point of search was within the current clearance area for delay so no further communication with ATC was required. During this time a point-and-click loiter (PCL) was initiated. This was done without double checking the altitude settings. The PCL was set to 15;000 ft. MSL. As the aircraft moved toward the area to search for the missing person the aircraft initiated a descent towards 15;000 ft. As comms slowed down with the ground agents; ZZZ Center called to verify Aircraft X's altitude. At this point the PIC edited the PCL to climb from its current setting of 15;000 ft. to the assigned altitude of FL210. Aircraft X's lowest altitude during this time was 16;000 ft. MSL.