Wrong on both counts.
You put way to much sophistication into something simple.
If terrorist want to randomly kill people on the street there are endless ways to do it without using cartoon characters to needlessly provide cover.
Back to the trash can on the street - or parked car - or planter with bushes in it - or any number of other hiding places they can put real explosive is real quantities that can cause real damage. It is brutally easy to kill us if you have the will and the explosives to do it.
You will not win if the only way to win is to remove the hiding places - they are endless.
Cheap electronic signs are nowhere on the list of requirements to do it.
Wrong on both counts? Do you mean someone actually noticed, and stopped the 9/11 terrorists? Darn, I didn't hear about that. Here all this time, I thought they succeeded.