Two main possibilities:

1. There is a bad employee who decided to keep the camera for themself.
2. It's possible that the car, being rented for such a short time was not fully inspected and was transferred to a new renter and who decided to keep the valuable equipment.

Either way, chances of you getting it back are pretty low, unless it shows up. Unfortunately, I think your only option is to file a police report reporting it lost/stolen and then file an insurance claim.