How much does an engine seal leak?

Hello!

I have a Ford Focus ZX3 2001 Hatch 65,000 miles ( 4 cyc - I think) I have a leak in my valve gaskette on top of my engine that I am getting fixed. However they think I have one underneath also- engine seal. They are going to run dye through it to check. I am hoping that the oil up top is just running down the engine and burning off down there. My question is wouldn't I have a small leak on the ground if I was leaking down there? I haven't had any loss of oil, I have been checking and I last had the oil changed in Jan. My engine oil is still reading full like when I had it changed. Even with the small explosions I seem to be having on top, I am not losing much oil. Is it possible to leak down there like that and not be able to see it on the ground? I have not seen any and I have been watching. I can only smell the oil burning as it hits the engine. This doesn't happen all the time. It's periodic. Thanks!