I figure it this way: The first time the truck was searched, they had not yet found Dylan's remains. Now they have part of Dylan's shirt, either his shoelaces or one shoe, and a sock. What if there was an unknown fiber(s) originally found in the truck, but there was nothing to match it to? What if that fiber matches up to one of those items of Dylan's, and Mark said that Dylan had not been in the truck. Nothing problematic about that scenario. Everything was found when it became available.
Or what if there were some fibers or evidence found with/in/on/attached to/whatever any of the items of Dylan's that were found, and it matched with something found in/on the truck in the original search(carpet, unusual dirt, insect eggs, what have you). Nothing problematic there either.
There might be a presumptive test for blood, but then it has to be confirmed to actually BE blood and then Dylan's blood. Once confirmed, there might be cause to go back and see if there is anything more. Nothing problematic in that scenario either, as it would be done in the order it was supposed to be done in, with no way for anyone to plant additional blood (because who would have a vial of blood of a deceased young man who came off the mountain as a few bones and scraps of clothing?).
There could be some circumstances where evidence might be questionable, but I see far more scenarios where any evidence, if found, would not have any questions involved. JMO.