re: Home question: what is this and why is this light on?

Not exactly related, but this post reminded me of something I saw back when I used to rent a house near LSU.
There were some apartments across the way that one day nearly burned down after some catastrophic failure at the service entrance. Entergy came out and did some work, which included installing some box of equipment that had a red light and buzzer attached. At least several times a day, these indicators would indicate...something, sounding for 10 seconds or so with the light on.
Anybody know what this might have been? I always felt sorry for those apartment dwellers—I could clearly hear it sounding a couple hundred feet away.