Another idea if it's not the above is a small freeze plug leak. I had been slowly loosing coolant for a while on my Jeep at one time. When I pulled the tranny, clutch and flywheel to do a clutch, I noticed the rear freeze plug was leaking. The coolant then evaporated off the block before it could drip. Unfortunatly, without pulling the flywheel off, this just looked like a rear main seal starting to leak. But there are other freeze plugs you could check.