Part 107 UAS pilot reported during a test flight the UAS pilot began searching for an approaching aircraft and lost VLOS with the UAS going behind a cloud. The UAS pilot regained VLOS and landed the UAS.

Narrative

Pilot In-Command (PIC) operating alone as Pilot Manipulating Controls (PMC)/Visual Observer (VO) had completed two previous manually flown test hops with a new aircraft (sUAS) following employer commissioning checklists for new makes/models. On the third hop the flight test was a simple area waypoint mission within 250M of the flight deck to evaluate the in-flight behavior and make the pilot aware of normal patterns flown on this type of mission using specific automated flight tools.During this mission the aircraft executes a series of 'calibration' maneuvers that are unique to this sensor payload; and the attention of the PIC needs to be on the sUAS so as to prevent loss of line of sight/visual reference.While the sUAS was performing this maneuver; a small aircraft was heard in the distance about where the closest airport is located; and that aircraft was not visible. PIC was in a condition of scanning for traffic while also maintaining situational awareness and line of sight to the sUAS. This additional workload factor interrupted the PIC from conducting normal full 360 scans for about a min while the scans focused on looking for traffic while also maintaining line of sight to the sUAS.During that time; moving at the speed of the winds (280 at 12 knots) a low coastal cloud not visible over the 30M high tree line 50M west of the flight deck drifted into the operating area and the sUAS entered a flight condition less than prescribed by FAR 107.51 (c) and/or (d). PIC terminated the autonomous flight test and manually flew the sUAS directly to the Landing Zone (LZ) from a point 50M AGL and 220M away. PIC elected to manually fly and not use the predefined 'return to home' feature since that would have flown the aircraft up to 60M which would have further limited the visual reference ability of the PIC. While the sUAS was visible to the PIC the entire time it was airborne; the operation clearly was not FAR 107.51 compliant for about 1 min 30 seconds at the end of the mission.