My 1992 245 has a habit of losing coolant, as much as the whole expansion tank (2 quarts?) during a 120 mile drive. I can't see any sign of drips or smell any coolant in the cabin, carpet is dry. Has been to my mechanic's twice, and they are starting to say maybe it's just normal loss of coolant. They pressurized the system, and said it held pressure for two days. No water in the oil or vice versa. So it seems that wherever the leak is, it happens only while driving, perhaps under load. A little hard to tell because the mechanic has only the streets of Brooklyn to test drive, when the leak seems to happen more at highway speeds. Any thoughts or clues?

Thank you.