Do security companies only monitor alarms or do they also monitor cameras...?

So I am interning with the United States Geological Survey and last week I got back from field work late. I had to go into the office but the alarm had already been set. (it looked like people were still there at the time) I didn't stay I had to go find a bathroom then I came back and I didn't see anyone show up. Today I heard them talking about the incident in the context that the alarm went off randomly. With it being a federal office there are those CCTV camera's overlooking the entrances. Nobody has mentioned anything to me about it. I've told a couple other USGS employees and they just laughed at me, lol.