I'm wondering if MTR went back for the shoes, why he would remove his, but not TLM's. One could speculate all sorts of things about this, but IMO, he would have taken away both pairs.
It's too bad Lillian couldn't remember the exact date of when she found the shoes. The closer to the abduction date, the less likely MTR had time to look and find them.
They threw other clothes and evidence into a dumpster and garbage can at the car wash. It must have seemed like a sound decision at the time. MTR would have had to go back to retrieve those pretty quickly, as those containers get emptied often. Daily? Every second day? Weekly? Regardless, I think he was safer leaving them there than being spotted dumpster diving, opening garbage bags at a car wash. That would be pretty conspicuous (and disgusting) and he'd have a lot to answer for if questioned.
It's my personal opinion only that MTR did not go back to retrieve any evidence. Why make the decision to throw shoes out the car window and other evidence into the garbage at the car wash, only to risk going back to find them? Then what? He'd just have to dispose of them again.
JMO