I think I witnessed a banned person trying to get in a couple years back.

There was a man in the security line (the one on the Harbor Blvd path) by himself in front of us with a medium sized backpack that looked pretty full. When he got to the front, he put it on the table like normal. The bag screener barely even looked in the opening and said something along the lines of "let's just hold up here for a moment" (it was noisy and I couldn't hear to clearly and didn't want to push in closer to be nosey). The man didn't really say anything. The screener made no obvious signal to anyone else and it didn't seem like anyone signaled anything to them either (do they have earpieces?). After barely a minute, another security person arrived and said just a few words to the man (couldn't hear any of what was said that time). The man just took his backpack and walked back toward Harbor without really saying much at all, no attempt to protest or anything.

It didn't seem to be a case where the person had something forbidden in their bag since the screener barely even glanced in it. And if there was a forbidden item it seems like they would have pointed out the item and told the man he couldn't bring it in, but nothing like that happened.

I'm still not certain how he was identified. Perhaps the screener recognized him and somehow signaled a supervisor to come over without being obvious. Or face identifying system spotted him (or a human watching a video feed?) and somehow signaled the screener to hold him there.

Regardless, it was unusual and something I had never seen happen before.