When parked it leaks coolant

Thank you for your quick response.
Yes, that is the cap I referred to.
I am going to replace the thermostat in a little bit but I'd like to tell you what happened today. It has left me baffled. Before I read your response, I replaced the 'looser' reservoir tank cap with a tighter (sealing tighter) cap and the Nissan definitely overheated with the temperature gauge going nearly to the red zone before I was able to shut it down. I lost most of the fluid again. I want to be clear that it did not reach the red zone. Once the engine cooled down I refilled the radiator and switched the reservoir cap once again to the 'looser' one. I then drove about 10 minutes to the parts store. Once parked, I lost fluid but only about a quart, if that. I topped off the fluid and drove maybe 10 minutes more to the gas station. I should say I did not use the A/C to the parts store or gas station. Also, the temperature gauge didn't move out of its normal range to either location. After shutting the engine down at the gas station I checked for the usual coolant pool under the SUV - nothing, it was bone dry, no loss of fluid. I then drove back home, without A/C, about 30 minutes, mostly highway speeds, and shut it down at home, again, no coolant loss. I'm super baffled. Could something have been stuck and then freed up? If so, what could it have been - I suppose the most logical could be the thermostat?

I am so sorry I didn't get back you sooner, some things came up demanding my attention.
I replaced the thermostat and since I have had no overheating or any coolant loss.
Thank you very very much for your help!