When the person who lived in your apartment before you moved in keeps the key then comes back shortly after you move in and steals small items of seemingly little consequence such as a pair of socks, camera memory card, package of hotdogs, or a pocket knife when plenty of things of much greater value are lying around. They may also leave a telltale sign that something is amiss such as leaving the light switch on, but turning the lamp off.
Aaron: Hey, Emily, were you in my room at all today?
Emily: Nope, why?
Aaron: Cuz someone turned off my lamp, but not the light and my camera memory cards are missing.
Emily: Darn previous tenant mischief