With scan card access system, the security guards need to scan their passes when doing the rounds.

Pretty easy to look at their scan logs and infer what they were doing.

Using bar codes seems relatively easy to fake, but if you have the infrastructure in place then the access scanner seems to address that concern. Main issue is having the access control in the first place - won't help you if the guards are 'inside' the scanner and would never need to swipe while doing their rounds.

At my work you have to scan to use the lifts, so you can see how long the guards take to do each floor in the logs.