2000 Renault Scenic new water pump and cambelt kit - but leaking coolant

If you have a coolant leak but nothing is leaking when checked it is normally possible to see leakage stains around the effected areas, i suspect renault are not trying hard enough, have a word with a local independant garage who will value you trade.
In the meen time place some card board under the engine and check it in the morning after the car has been stood. don't forget to check the heater matrix inside the car for leaks aswel

2000 RENAULT SCENIC RT 1.6 16V AUTOMATIC
new water pump and cambelt kit - but leaking coolant
THE CAR RUNS VERY WELL NO OVER HEATING OF ANY KIND
TEMP' IS STEADY AND ON THE BUTTON
BUT NOW AND AGAIN
NOW THIS ONLY HAPPENS WHEN THE ENGINE IS LEFT OVER NIGHT TO COOL RIGHT DOWN
I FIND ANTIFREEZE/COOLANT LEAK
UNDER THE DRIVERS SIDE
I TOOK THE CAR TO (BY 2 GARAGES)
THEY BOTH RAN THE ENGINE FOR A GOOD WHILE - BROUGHT UP TO HIGH TEMP' THEN LEFT TO COOL DOWN NO LEAKS WERE FOUND ANYWHERE FROM HOSES OR AROUND THE HEAD GASKET OR WATER PUMP NOTHING LEAKING
THEN I TOOK IT TO
A RENAULT DEALER TO HAVE BOTH A DIAGNOSTIC AND PRESAURE TEST NO LEAKS OF ANY KIND WERE FOUND
ALL I WAS TOLD AFTER PAYING THE DIAGNOSTIC FEE
IT LOOKS TO BE
EITHER THE HEAD GASKET
OR A
CYLINDER HEAD PROBLEM
YET THEY AGREE THAT THE CAR IS RUNNING VERY WELL
PLEASE CAN SOMEONE TELL ME
WHERE THE HEATER METRIX ON MY RENAULT SCENIC IS

Ok it won't be the heater matrix if the coolant is coming from under the car, the only thing you can do is leave it with the garage to have it pressurized over night. as mentioned before there should be telltale signs, coolant will leave traces from where ever it is leaking