Last night with a few friends over, $400 dollars was stolen..Has anyone you trust stolen money/something valuable from you?

Here's a few details. My girlfriend and I had 6 people over last night, did some beer pong and such. This includes, three friends we've known for years (2 of them, 15 yrs and other 4 yrs) and another guy ive known for 3yrs with his friend we met 2 months ago. My younger sister, 13, was over just on the computer (not drinking obviously). Before anyone came over, i moved my GF's money and just put on our bed, kinda stupid of me...but i figured no one goes in our room anyway and i know everyone there. Sometime during the night her money, $400 was apparently stolen (2 weeks of work for her.) We didn't realize this until the morning. We of course questioned everyone and everyone denied knowing anything or doing anything. My little sister claims to have seen one person go into our room, but him and his friend say they didnt. I had to question her a couple times to get a for sure answer so i'm not 100% sure she is reliable. With no one saying they did anything, we're at a loss..

*let me add in, when my gf texted asking the guy who my sister said did it, he was pretty mad "fuck you, fuck his sister, i'm a grown man...can make my own money" we found that weird, but he later apologized.

So has anyone else ever had money/valuables stolen from people you believe you can trust?