1999 Other Oldsmobile Models coolant leak

This is for a 99 olds silhouette with 100,000 miles. It is losing coolant but I can't see exactly where. There is a puddle on the top of the engine just under the thermostate housing. I replaced the thermostat and installed a new gasket, checked for leaks and nothing is leaking there. After the van ran for a while (2 days) the temp started going up. After opening the hood I could here a hissing coming from the same area as the thermostat but it wasn't leaking and I still couldn't see where the puddle is getting its coolant from.