Don't you have to allow for probabilities?
Is there really, statistically, sufficient likelihood that it signifies a potential shooting to justify notifying 'authorities'? I really dont' think so. You have _one_ case of a shooter doing such a thing, and there are umpteen other cases of shooters walking in front doors or wearing shoes or hats any number of other irrelevant details you could fixate on.
Personally, i think I'd have judged that the chances it was a case of 'about to let a shooter in' to be so close to nil as to be not worth bothering with when compared with the, greater, risk involved in stirring up 'the authorities' unecessarily - because it might end up involving cops who seem to who have a bit of a track record of over-reacting and being trigger happy.
However, as long as these 'security' people weren't armed I'd say it wasn't a big deal either way. The far more plausible 'risk' was surely just that he was some rapscallion wanting to let in his pals to see the movie without paying, and it's your call whether you want to snitch on someone for that. Probably depends on how old and grumpy one has become, really [I'm not that old and embittered against the young, myself...yet].