involved in a drug-sniffing dog case but as I recall the exception from the warrant requirement derives from the "plain view" exception.

While I defer to Old Dorms Rule in these matters, I'll take a hack at it. You don't have an expectation of privacy in the drugs you left in plain view on your console, and similarly, allegedly, you don't have a privacy expectation in drug odors emanating from your suitcase in "plain smell" of the drug dog.

At that point of the dog alerting, the warrant exception of "probable cause plus exigent circumstances" (sometimes erroneously, per my old Crim Pro prof, Charlie Whitebread, called the "automobile exception" because it's a) broader than just automobiles and also b) narrower than, "because it's an automobile there's no need for a warrant" despite what the cops think) kicks in, the "exigent circumstances" derived from the easy transportability of the suitcase or backpack.

Seemed like horseshit to me back in the '70s, but we've moved so much further to the right since then that it seems almost quaint.