Yeah, they sure should find out where it is and why , it could just be any reason not directly related to a crime, that's all i'm saying. He did buy trashbags later and we don't know where the truck was until plate was scanned some time on the 27th..
The guy just noticed the clothes and toys, possibly because they were not in a throw away state. A tarp in a dumpster and at a gasstation specifically, i personally wouldn't think much about, these things have a tendancy to rip. It might have been there but he just had not registered it as odd.
And that still doesn't explain any crime as is does go with a story of cleaning out a car you just bought.
He also could have rolled her up in that to throw her in the trash of course, but it's just one of so many options, the tarp in itself not being there is not that odd imo, it could have been inside the cabin of the truck for all we know.