So who was that in the Bathroom?
The man said "Brill died when he was two". And he searched Dean for with a Bug detector. Who was that? FBI? or another NSA agent?
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The BOSS of this place 👊
Another NSA guy, to make him talk.
Strangely we never see him again...
When Dean is talking to Rachel in the park, and the surveillance team is dressed as hobos\reading the newspaper with the microphone in it, they pick up on the name "Brill".
Gabriel Byrne's character is a decoy they send to try and get information out of him.
Furthermore, when Dean refers to Byrne's character as Brill, he basically confirms the name for the NSA guys, as Gene Hackman later grills him on the hotel roof: "Did you say it or did he say it?" and then Dean realizes that he said it himself.