There's a parking lot across Airport Way S from Elo's Philly Grill. It's visible from the I5 S ramp to the Spokane Street viaduct.
It's been used for years by Snohomish County Transit busses laying over.
In the last few months, I've seen a fleet of Smart Cars parked there. They either look like they have Snohomish County Transit colors or still have white plastic shipping wrap, it's hard to tell from the ramp.
I never get down that way so I'm hoping someone has gotten a closer look, or better yet has some knowledge of why they are there.