Doubttful it was a FAM for reasons stated in previous posts; but why are
you concerned to begin with?

FAMs haven't need to go airside via regular security checkpoints. And most FEDERAL LEOs don't announce themselves within earshot of the general public, let alone use regular security checkpoints (generally).

Spiff does have a point about DCA though.

Some days you have a literal army of "armed with weapon" passing through certain concourses... and NOT all armed FEDS travelling via DCA are common LEOS (in the sense of the word normally in use) as an ordinary citizen might think.