ISO27001 The information security standard calls for building security to be part of the overall system.

Most companies will have some security on the front door; it could be a fully manned reception desk or a keypad entry system or even a locked door. Anyone intent on gaining unauthorised access will usually target another entry point. This could be an insecure window or even better a rear door or fire door that has been left ajar for those that smoke.

I have seen some quite secure buildings which are neglecting the "back door".

In the warmer months of the year companies that do not have air conditioning often prop open rear doors to allow for better air circulation.

If no one is watching a thief or data gatherer can simply walk in.