Sometimes things get stolen even when cameras are everywhere because it is not too hard to wear a fake mustache, mask, sunglasses, and a hat where it is difficult to prove the identity of a robber.
If you know magic tricks, it maybe possible to steal something where it is hard for the camera to pick up by tricking the viewer of the security videos.
Some cleaning staff may throw away stuff like pens and pencils, and steal it by secretly putting it in their pocket when they reach an area without cameras like the bathroom.