What could be the cause of Water running in Condo?
It's quite possible that some other unit is draining into the same pipe. Typically water sounds are generated only in the drain pipes, because the inlets are systolic (i.e. always at peak pressure).  Since at least so far nobody's reported leaks or water damage, my uneducated guess is that some faucet or toilet valve gave out, and there's constant water flow within the system.  Whoever lives in the unit where this happened may simply be unaware of (or doesn't give a darn) whatever plumbing fixture has water running into and out of it.