packwood,
did you already contact WDFW? With these situations it is important that you contact them immediately. It is unbelievable how many times people tell WDFW Officers about "suspected" poaching 2/3 weeks or a month after they observed the poached animal/incident, an officer can't really do anything with that and the info is pretty much useless.
Also, the area you listed is within the treaty hunting grounds of the Nisqually, Puyallup, Squaxin Island and Muckleshoot Tribes. And I am sure you know, but that area is hunted heavily by the Puyallups and Nisqually's. So it might actually be a completely legal kill. Some tribes do not have a wastage law, so it is actually legal for them to shoot and take the horns and no meat. It is up the to the actual tribe to decide if they want a wastage regulation.
Hopefully you already contacted WDFW about the incident. Because quite honeslty 18 days after the fact is really no help to an officer. With suspected poachings officers need to hear the info immediately.