Mystery - Oil Leak

Hi

For the last few weeks I have been noticing that my engine oil pressure gets low, runs fine for a week after the re-filling. There are no visible oil leaks spots under the car. I get my oil change done at Jiffy Lubes and they suggested that I might need new rear main seal or oil pan gasket. They are not sure. I am surprised that there are no visble oil spots under the car or on the gaskets. Could it be that oil is being consumed by the engine. I could smell a litle oil today morning when I started the car. But I donot know if that could be due to a cold start. As the smell subsided lately and there is no visible smoke coming out.

Any pointers are welcome.

Regards
Vishal