Its a leakage.
If its external wall - seal the cracks in plaster, it will resolve it.
If there is wc or bath on other side. Do check for plumbing line leakages first at cp fittings then at pipe joints (better way is to close the supply of water and watch it for fe days, if its get dries off then it is causing through plumbing line leakage)
If its not through inlet water then do check for is water getting accumulated below flooring tile, if after removing tile, water is seen in bedding then, ensure all joints in flooring are sealed correctly.
If still issue wont get solved then you need to broke the waterproofing and redo it.
You can also try passive socketing to arrest leakage, before breaking waterproofing.