I had "An Insidious Trap" come up just now during a game, and I played it that as per the Lurker rules (even though I wasn't using the Lurker expansion in this game) the gate can open in the street, and be closed, but never sealed.
I also found the new Mythos card "Private Security Hired in Uptown!" to be singularly odd. It opens a gate at the Woods (an Uptown Location), spawning x monster(s), then after movement, removes all monsters in Uptown streets and locations back to the cup. So unless a fast monster comes out, and matches the movement symbols (for the record, only 5 of the 8 total fast monsters do match) to move twice out of Uptown, any monster that does spawn will be removed that same turn!