Water leak marks visible from room below bathroom, how to find the root cause?

[larrycan]

We can see these marks below two bathrooms in the floor above. Picture attached. Any ideas what could be causing it ?

 

[Mr_David]

Somethings leaking. You could poke a couple holes with an ice pick and then use various fixtures one at a time with a wait period between each test. If you can’t find anything visible in bathrooms you may have to cut holes.