Burglar

Burglars in The Sims Often, your house will be infiltrated by a burglar, who will rush round your house cramming items the size of television sets into a small black bag. Burglars only appear if every Sim in the house is fast asleep, and can easily be prevented by installing a burglar alarm at every door. If one is installed and a burglar still attempts to rob you, the police will come running, arrest the burglar and offer you $1000, as well as being reimbursed for anything the burglar may have stolen. However, if they aren't caught, they simply run off with your Sim's possessions.

In The Sims 2 some sims, particularly fortune sims often have "getting something stolen" as a fear

An exploit to burglar proof your house that works in both games is to make a single wall on both sides of the street and put an alarm on them, causing the alarm to be set off as soon as they appear on the lot. Its also an easy way to randomly make 1000 simoleons, especially if you make sure all of your sims go to sleep at the same time.