the local hardware stores or plumbing supply stores (how's that for high tech) sell such sensors. About the size of a deck of cards, with a wire and a sensor at the end, a 9V battery and serious sound effects when wet. Here at the mansion
we have one such monitor tied to the central alarm in case the sump pumps fail. The standalone unit costs $10 or 12 or so...
If I may put a plug, Home Depot carries the Basement Watchdog alarm: SKU: #854911.
I have their $600 sump pump and it *rocks*
but then the computer room is 10 feet away from the pump pit.
The Basement Watchdog Features:
Minimizes the risk of water damage when placed near problem areas
Sounds an alarm when water touches the sensor, only 1/32" deep
Solid-state circuitry is extremely sensitive and reliable
110dB alarm can be heard throughout the house
Alarm will sound up to three days
Battery will last 24-48 months
Place alarm on the floor, under sinks, near water heaters, in laundry rooms
Sensor has a 6' cord so it can be extended for hard to reach spaces
Requires a 9-volt alkaline battery (sold separately)