I don't think we're that far apart doctorquant. My point in the other thread was that police didn't have probable cause to arrest him at that moment because all they had was this odd-looking device and the kid's word that it was a clock. They didn't have anything to indicate that he had intended it to be a hoax bomb, and it was missing one of the key pieces to make it appear as a prima facie bomb - the explosive element.
Just because they didn't have probable cause to arrest in that moment would not have stopped them from continuing to investigate. If they turned up evidence that he had intent for it to be something else, they could have taken action then. Police do that sort of followup investigation all the time after making a decision not to charge. I'm not sure they could ever get there, absent a confession, but they could have tried. A thorough investigation may have turned up evidence of his intent - whether that was to take apart and re-case a clock or to scare the crap out of a teacher - but it didn't occur.
I think I did say initially that I was skeptical about whether they even had enough to get to reasonable articulable suspicion to investigate, but that was before I read the hoax bomb statute. They didn't have RAS for a real bomb because it was pretty evident that it wasn't one, but I think they do have it under the hoax bomb statute.