“how come the theater manager knew he was carrying?”
OPEN carry.
You are right. I did not thoroughly read the original article. But let me say that if, then, he was carrying open, that in the mindset of the moment he may well institute a local theater policy not against those who open carry, but also against those who are licensed to carry concealed.
In PA, it is a relatively simple thing to do to obtain a CCDW for any PA resident, or for a resident of an adjoining state licensed in his own state. If you decide to open carry, you can expect to be stopped and questioned (harassed) by local police to show that you are not a person prohibited.
This kid, whatever his point, is still exhibiting poor judgment in advancing the interests of other carriers.(IMHO)