This originated in Cheyenne, which is Union Pacific territory. Might have been on UP the whole trip.
The railroad(s) involved will know exactly where the car went, where it stopped, how fast it was moving, etc. Record keeping is pretty good these days.
Back in the days of Penn Central (1970's), they lost cars and even entire trains.
But those times are over.
If one of the bays had a leak, I'd start looking the entire route the car traveled.
If it got "humped" at an intermediate yard, I'd look there, too -- car may have leaked when it hit the standing cars after going "over the hump and into the bowl".
I can't see the car having been secretly placed on a track somewhere and unloaded that way. The railroad would know about that, and crews wouldn't be able to do it, anyway -- not on a "through freight".
My GUESS is that the contents just leaked out along the way...