The movies

When I was still a mid-teen girl, my boy friend and I would go to our local cinema (this is UK) where they used to have twin seats along the back row, like a small sofa. As soon as the lights went down the couples would get to work. We never had actual sex, I don't think that could have been done but I was usually fingered as I gave him a wank. When he would tell me that he was going to cum I'd wrap a handkerchief around the end then give a few more rubs as he shot into it. When I got home I'd just drop the handkerchief into the laundry basket. It never occurred to me that my mother would notice anything unusual. Luckily she never did, or at least never said anything.

I'm sure that she noticed. Dried cum on a handkerchief after you've been out with your boyfriend and the smell would have given it away. Unless you had a teen brother in the house it wouldn't take much effort to work out where it came from. If she ever reminded you to be careful then you can be certain that she was aware. She would have been happier that you were wanking him off rather than actual sex. Don't forget that she was young once!