how did they find out?

I'm 15 and my boyfriend of almost a year and I have been messing around physically (you know, a lot of touching and feeling along with a lot of kissing) every now and then for about the past 3 weeks. His parents found out and we have no idea how. His mom thought that my mom deserved to know about this and so they got together and talked about the whole thing. My mom pretty much freaked out about the whole thing and I got into a lot of trouble. My boyfriend and I have no idea how his parents (probably his mom) found out. It's almost impossible that she saw us so we figured that maybe she had gone through some of the letters that I wrote him but he looked in the Hallmark bag where he keeps all of the letters that I give him in and he said that everything was EXACTLY how he left it, with all of the cards that I gave him on the top and the letters underneath. The only time we talked about it on the phone was when we were at his dad's house and there was no one in the same room as my boyfriend when we were on the phone and all of the other phones in his house were hung up. Do you know how his parents found out? I also wanted to know if we'll have any more chances to mess around anymore because we both really want to. Thanks