Don't get complacent, just because you work at a "secure" site. It only takes one rogue legit visitor (say a contractor) to admit a criminal into a building, who could secrete themselves out of sight until everybody else has gone home, then nick whatever they wanted.
A few years ago at work, a load of very expensive electronic equipment got stolen from inside a locked room inside a building with electronic access control. The internal door to the room was burgled, but there was no signs of forced entry into the building externally. The consensus of opinion is that someone with legit access facilitated someone else without legit access at least through the external doors, who then broke into the room containing the equipment after working hours.