No, not at all. I've had it happen after leaving the machine much longer than that. The thing is it happens quite infrequently, and isn't behaviour I can reprod

No, not at all. I've had it happen after leaving the machine much longer than that. The thing is it happens quite infrequently, and isn't behaviour I can reproduce on demand, so I've not been able to reliably isolate the set of conditions that cause the problem. As I said in another comment, the first few times I thought I must simply have forgotten to lock the machine. But of course then I became paranoid about locking the machine and so when it continued to happen from time to time I realised something really wasn't quite right. Like I say though, it's just not behaviour I can reproduce on demand - super annoying. It's most likely some quirk of my machine configuration, or possibly a driver issue (though I tend to like to keep things up to date), but I have a super-unadventurous Dell XPS so it doesn't make a whole lot of sense.