Yesterday, in the process of checking out my truck ac, I put some dyed freon in the system. Today, I crawled under the truck, found oil on the frame, and after checking with the ultraviolet light, found it glowed bright green.
The dyed oil is from the system, and the reason it's on the undercarriage is that that's where the evaporator coil drain is right above that spot.
So, tomorrow the truck goes back into the shop that changed the evaporator coil over a year ago. Since it's such an odd event for the coil to fail, my contention is the coil was faulty, and they should repair it for free. We'll see.