@Akos4 Yes, many reports like this here on the forum. Same sort of thing happens to guests. Customer service has dropped off a cliff.
Try contacting them through their Twitter account rather than by phone, you might have better luck.
If you are sure the guest stole the keys on purpose, rather than just accidentally forgot to leave them, you should probably get the locks re-keyed and save the receipt so you can possibly make a claim for the cost of that.