When did this start?

[thebanjoranger]

I was just listening to U2 and realised that I was singing along. This would be fine except that I forgot there was someone else in the lab. Luckily she doesn't care, but what's worrying is that I just noticed I was singing along, I don't recall where I started singing, or thinking I'd start singing along.
Do other people do this? Are there other things you find yourself doing and then wonder when you started?