My theory, FWIW.
He had to have known he'd be the primary suspect, given his history with Venus and her parents. So he knew he'd have to have a "solid" alibi, and be no where near Michigan when she disappeared. His witnesses must have checked out, and must have been people who would not have a reason to lie for DS. Whoever they are, I believe he made sure he was seen at his job, in public places, with plenty of evidence he was in Newport News during that day. I'm willing to bet he made sure to stop at a bank/ATM and other places he knew would capture him on video. So I do believe his alibi (that he was in NN that day) IS solid. I think he made sure of that.
Because he knew he had a solid alibi that he was in NN that day, he got careless. That a second person is involved would also explain the truck being parked away from his apartment complex. I'd be interested in knowing if the complex has a security gate at the entrance. In any case, I don't think he'd risk having this person bring it back to the apartment lot on the chance that a neighbor or employee of the complex would see someone other than DS driving his vehicle. That's not some generic sedan that could be mistaken for any number of cars belonging to any number of tenants; it's a huge truck with a crew cab. It's going to be noticed, and people are going to notice who's driving it. I believe he planned for this person to leave the truck away from his apartment (but within walking distance) so that he could retrieve it, make sure it was *clean* of any potential evidence, and then bring it back to his apartment lot. That it was found by NN police a few blocks away tells me they got to it before he did, and that they did find it very early on.