What are the signs of a potential plumbing leak?

The signs of a potential plumbing leak often include water stains, which can indicate a buildup of moisture that may be coming from behind walls or ceilings where pipes are located. Unexplained increases in water bills are a critical indicator, as they suggest that water is being used without obvious cause; this can be a direct result of a leak going unnoticed. Additionally, the sound of running water, especially when no taps are in use, can signal that water is escaping from pipes due to a leak, which often leads to these other symptoms.

While damp spots in the yard and foul smells can indicate issues such as sewer leaks or improper drainage, they are not as definitive as the signs described in the correct option. Excessive condensation on pipes may suggest a temperature or humidity issue but does not necessarily mean that there is an actual leak. Blocked drainage systems can lead to backups and other problems but do not directly indicate a plumbing leak. Thus, the combination of water stains, increased bills, and running water sounds provides a clearer, more comprehensive picture of a potential leak situation.