I have a little bit of experience here, and the biggest real trouble is complacency. The second biggest is comms.
The first requires a blend of experienced locals that know the most likely places to look, and the points of ingress and egress to/from those places...
And folks that are not familiar with the area that will look at things with bright, new eyes. That's a hard mix to maintain, because complacency occurs in both those things over the course of time, and endless repetition across many shows/venues.
The second is very easily fixed with a comm channel that everyone stays off of, but that everyone is monitoring, that is there for direct comms, bottom to top, in case of a real threat. But that seldom occurs. comms are regimented through the various offices that control such a site. Even dispatch being able to relay takes way too much time, and creates confusion.