Where to find water leaks.
First floor apartment ceiling leaking.
A leaky roof can channel water anywhere along beams and rafters that may just happen to exit onto your ceiling.
You might want to go check for signs of moisture around and under your washer.
A refrigerator with a blocked defrost drain for example can lead to ice buildup causing melted water to leak out and seep into the floor and onto the ceiling of the lower floor.
Close up of flooded floor in the living room from water leak.
Screw type fittings especially in drain traps are more likely to leak than are glue type fittings.
An attic above an apartment may leak and deposit water anywhere onto a ceiling wall.
Owners are responsible for damage to their floor coverings ceiling coverings appliances.
The only way to define the issue is to start tracking the leak you can open up the ceiling to see wiht a flashlight and track back to the soruce from there or in uninsulated ceilings walls you can use about a 1 2 hole to inspect with a fiber optic camera rent for about 25 40 day from home depot or tool rental stores or some auto parts stores or buy at amazon o harbor freight or such.