Part 107 UAS pilot reported a lost link with a UAS. The UAS initiated an automated return to home and landed safely.

Narrative

We were conducting a series of drone flights in support of a science project at the school. Our position was just outside surface Class C airspace for ZZZ. The intent was to fly directly vertically from the top of our building; hover the drone at a fixed location directly above the building and at altitude of 375 ft above the roof; and measure temperature and humidity for 15-20 minutes before landing; replacing the battery; and launching again. We had successfully completed five uneventful flights over several days; with two of those successful flights occurring earlier today. In all previous flight; the positioning and control of the drone had been rock solid. On the third flight of the day; the drone was again commanded to ascend to 375 ft above the launch point; and around the time it reached that altitude; it suddenly reported a compass error that I had never seen before; and it yawed an unknown amount from its originally north heading. The student visual observer; who had briefly looked away; reported to me that it was no longer in its expected position directly over us. Shortly thereafter; the drone reported a problem with the GPS lock as well. Neither of us were able to determine visually in which direction the drone had moved; and the video feed from the downward-viewing camera that I had been partly relying on to monitor the drone's position over the building became very poor; with only sporadic updates. Very soon; the occasional video refreshes that we got showed that the drone had; uncommanded; moved well out over the Lake; or at least 0.5 NM away from our position. Because I had lost positional awareness; I did not execute any positioning commands whatsoever except yaw to try bring our building into view of the camera in the hope that I could manually navigate it back home. Before I could do that; the video feed became unusable; and in short order; the control link was lost altogether as well. Because of the reported GPS problem and because I didn't know why it had flown away; I was also reluctant to send a return-to-home command; because I didn't know whether the drone had reliable position information and whether it would return to the correct starting point.We had not been able to ascertain in exactly which direction it had gone or how far it had gone; but at some point the link was reestablished; and it became apparent that the drone was approaching from the northeast and automatically executing a return-to-home due to the lost link. We reacquired it visually when it was perhaps half a mile from us; and it then automatically landed again on the building roof without further incident.During the period when it wasn't under positive control; it busted the 400 ft AGL altitude limit; because it was no longer directly over our 150 ft tall building and was probably at around 525 ft relative to the surrounding terrain. And because of the direction from which it returned; we also suspect; but are not yet certain; that it busted the surface Class C airspace that begins just a few city blocks to our northeast. It's because of these two issues that I'm filing this report.I don't know why we experienced this problem after a number of uneventful flights; but I can't rule out radio frequency interference; especially given the compass error; loss of GPS lock; and poor video feed all occurring at nearly the same time. Our lapse in maintaining continuous visual contact was a contributing factor; because once it moved away from the expected position; we lost all situational awareness and were unable to effectively intervene to manually bring it back to the building.A factor in losing visual contact was that the drone had remained reliably directly above us; stable as a rock; for all previous flights; and it was strenuous to keep looking straight up for long periods of time. There was no better vantage point because of the confines of the roof; and there was never previously any difficulty spotting it again after briefly looking away; which led to complacency.