@Jim472
Hi Jim. The guest possibly would be embarrassed about it and probably doesn't know quite how to respond. If he was 50% of a couple it would not be seen as anything sinister, just something they might regularly use. But having left it behind would probably want to distance himself from it.
I have had a few personal things left behind in my time hosting, on each occasion I have felt the value of the item was not worth embarrassing the guest over and just thrown it out.
A USB drive....what are we talking about here Jim $3-4 ...personally I would not have mentioned it to them.
Cheers.....Rob