Upvote:2
Uniformed security at a church is uncommon enough that you can use it as a conversation opener.
If I were you I would seek out a (non-clergy) trustee / deacon / elder / congregation-council member / whatever and simply ask "what's up with the guards?" That should open the topic, and you will likely learn something about the congregation's history and struggles.
How do you find the person to talk to if you don't already know them? Easy: ask a person who serves coffee for an introduction.
Believe me: asking polite questions of lay leaders honors their offices; they'll love it. Don't hesitate.