Scott, I'm not very good at condolences -- as you'll see -- but felt
compelled to write since no one wrote of other possibilities.  You said Toon
was in her cage, in the shade, and that the car windows were open?   Ms.
Doran and C-pi were in New Mexico during the summer, which is hot hot.
Unless you live somewhere on the equator, I don't see a 6 mo. old ferret
overheating in March.  I'm no expert, but there seem to be greater likelihoods:
 
  Dalmations can be rambunctious ...  is it possible that she was stepped on?
  Could she have been mortally hurt in the park without your knowing it?
Ferrets are awfully tough; I've given mine unintentional whacks and they
don't show a sign as to whether they're really hurt.  And I don't check on
them every hour after an accidental kick.
  Also, was there anything in the car she could have eaten/drunk from the
cage:  brake fluid, coolant, oil etc?  I know the questions seem cold, but I
only ask them to find the cause.
  Nothing now does any good, but you didn't do anything bad.
[Posted in FML issue 1140]