Sister in laws panties

Ever since I was a young boy, I have been into sniffing used panties from girls I thought were hot. When my wife and I got married she has three sisisters, I have sniffed all their panties. All four (my Wife and her sisisters) have similar scent, but one of the Twins, the one that is the hottest of the three, really smells great. I have been using her's*s from 19 to now in her 40's.
I have a question, a few years ago I was going to do some work at her place. She wasn't going to be home, she told me "While you are here, don't go into My underwear drawer." I told her "That's okay they are no fun unless they have been worn." She winked at me and said "Oh yea?" The next day when I went over, there on her bed.. was her sexiest pair of panties, a lacy thong. It was quite apparent that she had masturbated in them, I was so turned on I had to jerk off right there in her room, cumming all over her panties.
Soon after that I had to back down the flirting with her because the wife was getting pissed off, thinking something was going on between us.
Well, fast forward a few years, I was going over to take care of the dog while they were out of town, in the hamper was the black pair in my photos, again it was quite apparent that she had masturbated in them.

Did she cum in them because she knew I would be sniffing them, or just because she needed to cum?