Welcome to the forum.
Have you traced that wire going down from the box? Maybe some kind of battery back-up for . . . ? Or a battery charger powered by the onboard 12-volt system? Are the batteries labeled "Rechargeable" ? The green label would be consistent with this IMO.
Good suggestions in the next post . . . door bell . . . any button on the exterior near the main entry door?
Maybe remove the batteries and see if there is any voltage coming in on that wire.The white label has "BATT" and "TRANS" on it suggesting that 12-volt or maybe 120-volt shore power is charging the batteries IMO. Is there a voltage stated on that label?
More clues please!
Let the music lift you up . . .
[Robert Frost reading live -- 2 minutes]