They may not have, but I find it difficult to believe that they knocked on ANY doors. Until we have a doc dump that shows the extent and breadth of the original investigation we may never know what they did and did not do in it's entirety. Heck, we still don't know what actually happened to Trayvon's phone since PD says they didn't have it, when it should have been collected as evidence at the crime scene, and the family (who would have no way of getting it out of evidence) claim that they didn't have it, and the only way the phone call from the girlfriend was found was in the call logs on that phone from Tracy's account.
It does seem to appear that for a PD that did nothing wrong, they did very little right either....we have rumors that they collected GZ's clothing, but we are reasonably sure they didn't test for drugs and alcohol, did they look at his hands at all? We just don't know...we do know that what they have for documentation of his injuries is woefully inadequate, so what else was inadequate?