I was anticipating another thread about the latest tragedy, but I'm glad to see this one brought back out. I will not, ever, make the mistake of not carrying in church. I care about those people too deeply not to offer my portion.
My church, oddly IMO, doesn't have any form of security detail. I'm part of the usher team, so I have reason to be standing, monitoring, moving through the crowd, directing people's movement, and I keep a conscious level of situational awareness. If I can keep an alert eye and spot something that is out of place or someone acting a bit too odd, that is the least I can offer.