I live in a two story condo in a four unit condo building. The lady who owns the condo behind mine (we share a wall and pipes) allowed a relative to move in. Because this lady is behind on her condo association dues, the association shut off the water several yeara ago. The relative that moved in has no water. I have constantly seen them filling up 1 gallon water jugs at another neighbors unit and taking them into their unit, presumbly to use for the toilet, wash, etc. However, in the past two weeks, i have had water seepage in my downstaira bathroom (our bathrooms are on the same wall) which has flooded my kitchen floor, which is right next ti the bathroom. I have asked them to please stop using their downstairs bathroom and they have obliged, but the water seepage continues. Prior to them living there, we had absolutely no plumbing issues. The seepage is coming from under the wall that we share. What could be causing this seepage?

Possibly a pinhole in a copper pipe in the wall, it's probably your pipe, not the neighbors.

The wall should be opened up and the problem found and resolved. There is a huge cost if allowed to continue. You already probably have mold in the walls and subfloor and ceiling. Don't mess around with this, it can get costly quickly.

Was your building originally built as a condo or was it an apartment converted into a condo? Has the condo association approved of the water being carried from one unit to another? I agree with previous poster. Open the wall and have a look-see or hire a licensed professional, a licensed plumber.

Best Wishes