Losing Coolant

I have a 94 Intrepid 3.3 litre engine with about 60k miles.
I am losing coolant but don't appear to have any leaks. A couple of months ago my water pump had to be replaced. About 6 weeks later I noticed that my temperature gauge was going higher than usual and it was taking longer for hot air to come out of the heater. I had the thermosat replaced and the coolant flushed. They noticed that my coolant level was down. They pressure tested the rad and everything appeared to be fine. I just checked my coolant again and it was down and I had to refill. Any ideas?

A failing head gasket? A minor leak in the cooling system? I had a leak in a radiator that was hard to detect because the coolant would boil off as it was leaking.

Thanks for the info. I thought that the pressure test would identify something like a leak in the hed gasket. I'm trying to get another year out of the car. Replacing a head gas wouldn't be worth it. Any idea if there are any additives that might stop the leak for a bout 6 months or so?

Bars leak, it is available at most autoparts stores.