It happens all the time on the Dash-8s. People are out on the tarmac at the same time and get lost.

There was a thread recently about the FAs checking BPs as people went on and how this annoyed some pax - shows the reason why it's a good idea to check and double-check!

As to it being a security breach, I don't really think so - the person would have already cleared security and be scanned as necessary for the country of arrival.