The puddle in the sink cabinet is the ending of a story — the beginning is somewhere above it, at one of exactly six places. Because water travels along pipes and cabinet floors before dropping, the puddle's location lies about the source. Ten minutes with a flashlight and dry paper towels reads the truth.
The method
Empty the cabinet, dry everything completely, and lay fresh paper towels across the floor. Then trigger each suspect separately while watching with the flashlight: run cold, then hot (supply-side leaks show now, and may need faucet-on pressure), fill and drain the basin (drain-side leaks show during the drain-down), run the dishwasher (its connection leaks only mid-cycle), and use the faucet sprayer (its hose leaks only in use). The first wet towel and the first bead of water on a fitting name your culprit.
Supply-side suspects: constant leaks
Shutoff valves and supply lines leak around the clock, pressurized — the puddle grows even overnight. Look for beading at the valve stems, the compression nuts, and along braided lines (a rusting braid is a failure announcement; replace it today, not eventually). Faucet-base leaks that run down the supply lines masquerade as line leaks — the towel test upstream at the faucet's underside separates them. Constant leaks are the urgent tier: they never rest, and a failing braided line can go from weep to spray.
Drain-side suspects: leaks only in use
A leak appearing only when water drains points to the trap's slip joints, the basket strainer's seal at the basin, or the disposal's connections. Watch during the drain-down: drips from the P-trap nuts mean re-seat or replace the washers (hand-tight plus a quarter turn — overtightening splits them); water tracking from the basin's underside at the drain hole means the strainer's putty seal has failed; drips from the disposal's ring or its dishwasher hose connection name themselves. Drain leaks pass gray water — slower emergencies, faster mold.
The two impostors, and when to call
Impostor one: the dishwasher's own door or internal leak, arriving via the shared cabinet floor. Impostor two: condensation on cold lines in humid weather, mimicking a fine leak — the towel wraps entire pipe lengths rather than pointing at a fitting. When to call instead: a shutoff that won't close (you have no way to service anything downstream of it safely), leaks inside the wall behind the cabinet, corroded metal traps that crumble at the touch, or a repair that returns. Lumin Drain NJ repairs under-sink everything across New Jersey — usually in one visit with parts on the truck: 929-662-1816.