Are you aware of any sump pumps in your home or in the neighbor's home? If a sump was connected to the drainage plumbing and you shared any pipes with a neighbor, the sound may be heard throughout the plumbing system even with your water turned off. Though it still doesn't answer why this started after the fire system pipe repair unless a drain valve on the fire system is leaking or was left open.
Another question. Is your fire system connected to a water supply for your town home only or is there a main shut of for all units in a mechanical room and they share the same system? I have to wonder if there is a RPZ backflow device that has had issues from the time of the repair on the fire system and may be discharging.
If this only happens in night time hours, those are the prime times for thermal expansion. The fire system company may have altered something in the piping. Without a thorough examination of the whole system (water and fire) it's going to be tough. These may be good questions for the plumber/fire system company, also.`