I wonder if the evidence was dumped more recently. I also wonder if they are looking in the right place.
Where they are looking is close to a riverside park, but there is another riverside park upstream (Maconaquah Park), where major landscaping projects had been underway in the past couple of years. We don't know how LE learned of this spot to search, but if KAK didn't dump whatever this evidence is and instead learned of it from someone else, maybe the informer wasn't sure of the precise location, or KAK misunderstood if he was told...?
ETA: Or perhaps KAK even intentionally misdirected LE to a place that is close to, but not exactly right with the hope they'd come up empty but he'd still get credit from them for assisting AND the person he ratted on wouldn't hold him accountable.