The dye test can locate a leak, but, even without it if there was a leak there should be residual trans fluid on the bottom of the car, somewhere. Granted that if there are other leaks, such as oil leaks, it might be a bit hard for an untrained eye to recognize what’s what.
Actually, I’m trying to process how a transmission would even work without fluid, much less survive for any miles at all...
Anyway, thank goodness the car is still working ok!
Yes, there must be a leak, because that car (any car) would’ve been un-driveable without it.