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The water pump's shaft seal leaking, as distinct from a pump that has lost its impeller. The seal is the one dynamic seal in the cooling system — the shaft turns whenever the engine runs — so it is a wear point and it usually fails before the bearing does.

The evidence is a weep hole. Water pumps have a small drain below the shaft, and coolant traces from it are the definitive sign: the seal has failed and is doing what it is designed to do rather than forcing coolant into the bearing. A crusted weep hole means it has been leaking for a while. On a belt-driven pump this is urgent for a second reason: coolant reaching the timing belt destroys it, so a weeping pump on a belt-driven engine is a timing belt job as well.