How do you know you weren't mean to see the number?
Sometimes things can appear to be more than they are. I once suspected b/f of being up to something as he put a pin on his phone and it was either alway in his pocket or he didn't bring it with him. I found out the pin once so looked at it and all I found was a a bunch of texts about football and calls to me, his mum, his mates, etc. I asked him why he put a pin on and was reminded of the fact his other phone got nicked and he got rinsed on the bill.