Water pump runs constantly (won't shut off) even with no fixture open

Almost always one of these three:

1. A pinhole leak somewhere in the line — even a tiny one keeps the pressure switch from ever reaching cutoff. Check under sinks, behind the water heater, and along any exposed line for damp spots.
2. Pressure switch itself failing — if you've checked for leaks and found none, the switch (or the accumulator tank if you have one) is the next suspect.
3. A fixture that isn't fully seated — toilet valves and outdoor shower/city-fill check valves are common culprits people forget to check.

Quick test: shut off every valve you can find (including the city-fill and any outdoor shower) and see if the pump still runs. That tells you leak-in-the-system vs. switch/pump problem fast.