Originally posted by suginami
I agree with Peter.
FWIW, it might also be a water pump going out. I had a Nissan Quest that was losing coolant, and I had no leak that I tell on my garage floor.
When I had it up in the air for an oil change, after the bottom plastic cover came off underneath the engine, you could see the bottom of the water pump was covered in dried, green coolant. The coolant leaked through the seep hole because the bearing was going out, and It leaked only when the engine was running.
When my coolant was leaking this is exactly what happened...it seeped out from my failing water pump and was evaporated instantly couldn't find the leak until it got worse by that time the head gaskette went also.