Thanks for the follow up suggestions. So far nothing has unpinned since, and the license has stayed put... but then given the current pattern, I'm not expecting it to do anything til the 19th (will report back in a couple days either way).
I do indeed use AVG tuneup, but have had it (and start10) for many many many years (and rarely change any settings), so if it is deleting stuff now, I have no idea why it would have changed to do so.
I turn pc off every night (fully, not hibernate or whatever), and turn back on again in the morning. License (and pins) have gone through many turn-off and turn-on sequences, and have only this past month, once a week, started to do this. Again, have used this software for 5+ years now, with no prior issues (and come to think of it, I did have an update for start10 few weeks back or so... mayhap that could have been a trigger? ).
Will let you all know in a couple days if it happens again