Well... it didn't seem related at the time, according to Serino, so it wasn't taken into evidence, or if taken, wasn't marked as evidence. They found it later, not the night of the incident, so it would be darn near impossible to prove it was 'there' that night.
They didn't have to. All they had to do was find Martin's fingerprints on it and perhaps his shoeprints behind the bushes.
However they released the body without processing it properly and getting the fingerprints that could trace that back to him and as we came to find out they lost his shoes as well. Coincidence???