Are these his mother's panties, or a woman you've married recently?
If this is a woman you married recently, I think this is fairly normal sexual behavior for an adolescent in a very sexualized household with a new woman.
It would be interesting to know the age difference between the 12 year old and the new wife.
I wouldn't be over concerned at this point. This is an age where he is learning about his sexuality, and the panties may just be part of it. He obviously knew it was wrong, and was embarrassed. What I would stress at this point is that the panties belonged to someone else, and he had no right to take them. He knows he did wrong, leave it at that for now. I doubt that you'll see this in the future. As his father just make sure the two of you are communicating when it comes to sex, and everything else in his young life. Take care.
I wouldn't worry about it. He is growing up. Talk to him and stay open with him. If its something important he will tell u. Don't look down on him if he is wearing them, or even if he has them for another reason. Its weird yes, but part of growing up is figuring out urself. Good luck.
My son did the same thing when he was 9. He was embarrassed -- but I didn't make a big deal -- just said he shouldn't mess with my wife's clothes and that was it. No big deal and I would forget about it.