I Stole My Friend's Lunch Money

Yesterday at work, I saw a ten-dollar bill sticking out of my friend Alex's lunchbox. It was just sitting there, tempting me like candy on a shelf. I knew I shouldn't take it, but I did. I stuffed it in my pocket and went to grab some coffee instead of my usual snack from the vending machine. Now every time I see Alex, I feel this weird mix of guilt and glee. It's not like we're poor or anything; I could have just given it back. But I didn't. And now everyone is going out for pizza tonight after work, but with my dwindling wallet, I'm probably gonna stay home. So here's the thing: did I really do something wrong? Should I tell Alex and risk looking like a fool who can't resist temptation, or should I just keep this little secret to myself? What would you guys do in my shoes?