After the Boston bombing, if I were a cop being told "someone posted a photo of explosives on Facebook," I would absolutely ask about pressure cookers even on a routine visit. Just to see what their reaction is.
Well, it is of course possible the question about "looking up pressure cooker bombs" came from thin air. Lucky coincidence. But, in light of the recent revelations, I think it is less probable than some NSA analyst flagging the terms "pressure cooker"+"backpack" and passing that info along to police departments. YMMV
that's easy to say after the 'successfull inception' of it, but honestly, what you said it's pure-nonsense... just like kicking a ball the wrong way to then scream "I knew it was gonna happen!"
I wrote a column for many years called "Police Blotter" and have some familiarity with law enforcement investigative techniques. This is what happens in real life, not on HN.