The tailpiece carries water from the sink outlet down to the trap. A leak there may appear only when water leaves the basin. A supply leak can drip even while the sink is idle, so note the timing and the highest wet point under the cabinet.
Check the visible route without moving fittings
If you have already seen the leak, note whether it starts directly under the drain outlet, at a joint lower down, or above the tailpiece. Water can travel along the pipe and drip from the trap even when the leak starts higher. Stop using the sink if it is wetting the cabinet.
Share photos and recent changes
Tell the plumber if the faucet, sink, or trap was recently replaced. The technician can check alignment, the outlet seal, and the connection to the trap. Ask for a drain test after the repair before putting stored items back in the cabinet.
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