I don't get particularly worried, my only concern is that they may come back and say "you were £x short", that would be tricky, I think if that ever happened I'd just ask for the money back and walk. So ideally they would count it in front of you, but if they do then that's a bit weird too. I now always put it in an envelope and most girls just check it looks about right and don't count it. The one time I did not use an envelope I genuinely by accident left £20 in my pocket, I was mortified and to this day I still don't think she believed it was an accident, despite seeing her several times since.