What if the "clue" at the end of S6 EP1 was...

The odd double-take Lisbon did on the street...before...she went in alone.

It's as if she recognized someone, but, not someone she is close too...I.e. Bertram, Haffner, Mccalister, Stiles or Smith...if it was them, wouldn't she have assumed they were there to help or at least she would of reminded re team of a sighting?

I think it MAY end up being a clue...

My bets on a "blast from the past". You remember Mr. Mars don't you?:). Second guess would be...yours an my favorite Walter mashburn! Soon enough I suppose