What does it mean when the air conditioner drain pan is dry but the exterior overflow drain is dripping?

If by "exterior overflow drain" you mean a pan external to the AC coil and ducting (often installed on a horizontal attic crawlspace unit), and if there's water filling that "exterior overflow drain," then it would be likely that the coil's own drain pan has formed a leak. If the evaporator coil is older than 10 years, it probably was made of sheet metal rather than plastic, and so it may have rusted through in a location not readily visible.