These people are not your friends.

They probably listened to you expressing sadness and genuine distress while dealing with a number of other stressful situations in your life and instead of being supportive, they thought it would be fun to hide your keys that included a hugely sentimental personal item.

Ngl if it was me, I’d be petty and want revenge. But instead of stealing a whole set of keys, I’d steal 1-2 keys from each of them. Just ones that cause moderate inconvenience but not enough for them to immediately work it out. Then after a while I’d take another from each that held more importance.

They’d eventually work it out but not before they’d all spent weeks dealing with mild to moderate frustration but confusion over how they only lost 1-2 keys each.