Stealth cars on I-75

I don't go on I-75 very often in North FL, but the last two times I
have been on 75, I have seen the same phenomenon: a three-car convoy of
dark-colored cars with Michigan plates. The cars have some kind of
fabric covering over their lower halves, apparently to protect or hide
something--like a lower-half car cover.
I will appreciate an explanation of what these cars are all about.
Thanks.
Don