@Wayne-TP 

Thanks for your response! On reflection, I believe the RV20 simply behaved like every other robot vacuum would have - it lost its bearings, ditched the map and went on a search for the docking station, leaving the room it was in in the process.

I've fixed this by removing items too close to the docking station which were in the "line of sight" when the RV20 returned, and the behavior has not recurred. The RV20 now always finds the docking station from the map.

I've had to make one further adjustment: There is a doorstep/room divider of approx. 1.5cm height between the mud room and the adjoining room. That's where I put the invisible wall, but I've had to place it closer to the room divider to avoid a situation where the RV20 is on the divider, then slides down on the wrong side of it and is unable to get back into the mud room.

Only one other example of strange behavior - I initiated emptying when the RV20 was docked, and it somehow left the docking station at the start of emptying. That only happened once, though, and I attribute it to the "cheap" design of the docking station (no "parking space" included). Again, no big deal at the particular price point.