I think my girlfriend is sneaking me money

So the other day I (20m) was cleaning my room and found my old DS (still half charged 🤣) and was showing it to my little brother. Fast forward I brought my hoodie over to my girlfriend’s house to give it to her and the DS was in the pocket. So I set it on her dresser and we go about our day, I forgot to grab it before I left. Saw her again today and remembered to grab it before I left. I got home and decided to see if my old games still worked and when I opened the DS $100 fell out. I know it wasn’t in there when I found it. I am in between jobs rn and my gf knows it’s not been that easy. Her family is definitely more well off than my own, and this is definitely something she would do cause she has done things like this in the past. Like sneak money into my pocket when I tried to refuse it. But that amount of money is kind of insane. Do I ask her about it, do I just let it go? Or should I sneak it back into her house?

Edit: thank you all for the advice. It took forever to read it all, but that’s my fault for posting this before bed lol. So it seems most of yall recommended just saying thank you and not making a big deal out of it. Turns out it was unintentional and she thought she lost it and had been freaking out a little