Expansion tank
An expansion tank can empty itself onto the mechanical-room floor.
Tanks on domestic heaters and boilers fail at the bladder or the threads. The puddle looks like a tank leak, a PRV leak, or a nearby copper pinhole depending on who glances first.
Name the tank in the photo.
Include the heater or boiler beside it so the next person does not hunt the wrong system.
Do not tap the tank to ‘see if it is empty’.
If the floor is wet, isolate with authority and call. Knocking a corroded fitting open makes it worse.
One tank failure is a repair. Repeated nearby pinholes are a pattern.
Keep them on separate lines in the building log.
Questions people ask
Is an expansion tank the same as a water heater leak?
No. The tank is a pressure device. The heater is a different vessel. Both can wet the same floor.
Can I replace it myself?
Only if you are authorized on that equipment. Most condo mechanical rooms are not occupant workshops.