I recently noticed water marks in the ceiling on the lower floor that is about 3-6 feet away from the tub above that floor. I had noticed similar leak several years ago, just painted it over and did not see any more water marks for several years. I cannot seem to find where it is coming from.

I have checked thoroughly behind the tub and there is no leak in the fittings. I have let the water run. Only things we recently did was to clear the slow drain with hot water, or soda and vinegar or the 10 minute drain cleaners or plunger. I noticed one waste pipe connected as a V about a feet or so after the trap but I do not know where that waste pipe goes as it goes between the floor (could it be it is going through wall to the vent on the roof?). The water leak does show from the direction that this waste pipe is going. There is a thin wall there on the 2nd floor only. But there is no plumbing in the house in that direction other than that waste pipe, have no idea how far that pipe is going or where it is terminating..

How do I solve this mystery and fix it? Thanks

Yeah I have almost the same thing and when I tore out the wall I could not find anything. I have been waiting for a leak detection specialist for three weeks. I can't finish because I don't know if there is a problem. My advice to you, find a plumber who specializes in leak detection, just so you know for sure.

Since you can get into the pipes that supply the tub, look closely for water marks on the tub, especially where the over flow connects to the tub. Another place to check it if you have a soap dish or a grab bar, check the grouting around these to make sure they don't leak, you might even spray some water on them as you are watching under the tub. Look in the corners of your shower to see if there are any cracks that need to be filled with tub and shower caulk. These leaks are sometimes very hard to find. One other thing, what kind of shower door do you have? Sometimes the seal under them will go bad and need to be replaced. Also it is possible that one of your family members is getting out of the shower very wet and dripping more water on the floor than it can hold and it is leaking through.

Great advice from Letmetellu. One thing about water, water, like electricity, will seek the path of least resistance. What this means to you is that you may have a leak in one spot and the water not show up for a number of feet away.
Since you say, " I have checked thoroughly behind the tub and there is no leak in the fittings."
That would point me to the tile grout or shower door. Watch where the stream hits as you take a shower and check to see if the tile grout hasn't shrunk allowing water to get behind the tub. Good luck, Tom

Thank you Speedball1 and Letmetellu. I do not have soap dish or handle bar. The grout was redone but I will recheck. The tub door was replaced and is well sealed. I have checked for water marks behind tub near and around overflow pipe and drain, it is dry, I have checked with flashlight too. I will try to see the grout again for looseness but I had that redone not too long ago.

Could it be that the waste pipe that I don't know where it is going is really going to vent on the roof and the leak is from the roof from the sides of the vent pipe or condensation, etc. It is very small leak and just made water mark on the sealing but does not look wet. Etc?

SXG: Did you find out the reason after all? I have the same problem.