I would suggest doing a pressure test on the cooling system. You can get a tester on loan from places like Autozone or O'Rielly's. Test it at no more than about 15 psi. You just want to see if there are any leaks in the system.

Bcred is right... unless it is just steam, there has to me a smell of some sort. Oil is easy enough to distinguish. Coolant would have a slightly sweet smell to it. From your descriptions, I am inclined to say this is probably a coolant leak of some sort. Small leaks don't show up immediately as the coolant has to drop to a certain level before a warning will show up - like a COOLANT LOW warning, etc.

Also, you say once you open the hood you can't seem to trace it.... coolant leaks clear faster than oil leaks. Look around areas like the back of the engine towards your heater core and the auxiliary water pump for traces of dried coolant or leaks.

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