Or, it might be exactly as hp_fa said. The clue is that the non-rev was told to take any available seat. If we are very close to scheduled departure time, and there are seats available for non-revs (or revenue standbys), we frequently board them without issuing a boarding pass rather than take the time. The computer is updated to show them on-board, but I may be in 7D and you are in 28E while the computer shows the reverse.
Otherwise the non-rev (or revenue standby) is always issued a boarding pass. I don't know about LCC, but at AA there is a policy that states that passengers (revenue and non-revenue) who fail to respond to a page 15 minutes before departure may have their seats released to standbys. In a number of stations, this information is posted at the gate podium.