Mysterious Coolant Leak

For the past 2 or so months I have been losing coolant on my '96 Cutlass Supreme. My mechanic did a pressure test and discovered a bad water pump and timing belt cover. I went ahead a got him to change both. I got the car back a Monday drove it around for a bit everything seemed fine... By the weekend I had to drive the car on the highway for 350kms (220miles)... I was in stop and go traffic for a bit and the low coolant light came on, however the temperature gauge of the engine was perfect so I thought nothing of it.. When I got to my destination the reservoir was pretty much empty... So I topped it up to the cool mark and did not drive the car for a few days. I had to drive back home 350kms (220miles) and right away the low coolant light came on, when I stopped for gas I topped up the coolant again and went on my way. The light came back on and the engine started to get hot... So I pulled over and got it towed back to my mechanic. He did another pressure test and couldn't find anything, he then took it for a drive for about 30mins, did another pressure test and found a pin hole leak at one of the freeze plugs (he called it a welsh plug). He fixed the issue and everything seemed fine. The next day in the morning the low coolant light came on again... I topped it up and drove it on the highway for 400kms (250miles)... The light did not come on... The next morning I checked the reservoir it was full and the rad, it was full... The low coolant light came on again and about 10 minutes into the drive it turned off. I drove the car for about 160kms (100miles) no issue with the engine temp or coolant light coming on... This morning I parked in a garage with some cardboard underneath and had a few drips of red coolant... The reservoir was a bit low and however the rad was full... When I cranked on the car the low coolant light was on again.

I don't think the car is losing coolant from a blown head or lower gaskets. Any ideas where it might be going? It always seems that low coolant light comes on when the car has not been used in a few days.

What engine??

The only engine in this car, I think, was the 3.1.....there is no timing belt cover, since this has a timing chain.....I've never seen this leak coolant at the timing cover......

As for the w/p, possibly the easiest of all to replace.....whenever the system is opened, it has to be "properly" burped of all air, before it is given back to the customer......

For you to be adding all that coolant, sounds like the mechanic dropped the ball....these engines are known for lower intake gaskets, and if allowed to leak too much, can cause head gasket problems....

If I remember right, the coolant level sensor is located in the radiator.....if the light is coming on and the radiator is full, it could need replacement.....they become contaminated and also need a good coolant flush...

But keep an eye on the level of the reservoir......always check it first thing in the morning before starting it....if it's dropping, and it was properly bled, there is another leak....

Make sure your passenger side floorboard is dry. If it's wet, your coolant is coming out of the heater core which is inside the car.