You might notice a bubble behind your wallpaper or paint.  If you press on it, it might be soft.  If you puncture it, you may see water come out of the bubble.  These are signs that there may be the sign of a leak which has already caused significant damage and it’s probably getting worse by the day.  A leak is probably above or behind this area.  The pipe will need to be repaired or replaced along with the drywall.  Mold is also a concern in these situations.