To the original poster: He wasn't a FAM. Most likely a LEO (law enforcement officer travelling with his weapon....) Really isn't that uncommon, although you'd probably prefer the agent didn't verbalize that fact....

To the guy who witnessed a passenger with a yellow slip: He was a jumpseater....

Had 28 Secret Service Agents on my plane last year going to Fargo. The other passengers on the plane never realized just how safe they were that morning.....