Weird neighbor. Should I be worried?

We don't know who lives below us. We've met every single neighbor in our apartment unit except for whoever is below. My bf and I have both seen all of our neighbors in the laundry room, near their front doors, near my front door, etc. We're all very friendly. I figured it was bound to happen eventually through passing but nope. The weirdest part to me is I never hear them. But we think theyre there because they usually have the AC running nonstop most days. Our previous neighbors below were very loud. We always heard them move room from room, their tv, and their yelling. I get that this person might be quiet and doesn't see the light of day. Whatevs.. But this morning I saw 5 police officers walk out of there. Their cars were right in front and they looked very serious. I'm not sure why there would need to be 5 of them... Is that weird or scary or am I being weird or scary