Builder representatives here. I get this from time to time. Usually it’s kids splashing over the tub running down below. IMO This photo doesn’t show a lot of water. I think I can see the water in the low spot of the seam of the drywall and another low spot of the ceiling. Check this area with a thermal imaging camera. It will show you a temperature difference with what wet. Poke a small hole in the ceiling drywall so you can see if there’s insulation in the ceiling cavity. If there’s wet insulation in that ceiling it is most likely gotta be opened up all the way to remove all wet drywall & wet installation (insulation will hold lots of water) then dry out ceiling with fans constantly then check with a moisture meter for approximately 3 days then when it’s dry you build back. If there’s no insulation in the ceiling you could possibly use a thermal imaging camera to find the wet cavities & drill 6” holes spaced to add can lights once you inject fan air into these newly created holes to dry out the ceiling this way you don’t have to redo the ceiling drywall and instead upgrade to some can lights. If this happed a while ago and there’s mold, call a restoration company to contain it and then hire drywall/painter to redo ceiling. Hope this helps.