More Fun With Coolant Leak on 1999 Pontiac Grand Prix

Hi all -- I've been having coolant leak issues for two years now and you've been very helpful, but I'm afraid I have to ask for advice once again.

Earlier this year a small coolant leak returned so we replaced the hoses and clamps, but still had some drops coming out near the drainage plug of the radiator. We just couldn't find the leak and were frustrated, so we used some sealant. It worked until very recently when the small leak returned. This time, we made an appointment for a mechanic, but didn't get there soon enough -- on the 9th the coolant just gushed out of the car all at once, and we discovered the radiator had cracked wide open.

The shop replaced the radiator on the 9th, but on the 12th the Low Coolant light came on. There had been no leaks as far as we could tell, nothing on the driveway and nothing we could find under the hood in any nooks or crannies. The shop said it might be air in the lines, so we "burped" the car and added more coolant.

Today, a week later, the Low Coolant light came on again. The tank was down to below the level for "cool" but wasn't completely empty, and the temperature had never gotten above 185F.

Unlike the time we had a crack in the intake manifold gasket, there were no drops of coolant around the engine, and we've seen no blue exhaust from coolant burning off like we did before. We can't hear anything dripping after we turn off the engine, either.

Where could the coolant be going?  Is it even possible to still be air in the lines? Any ideas as to what's going on? Thanks!