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maybe go tag the deer first before going to retrieve your gear.
They were able to gut and pack your deer out before you got to it? I don't think they stole it, I bet they thought someone left it or didn't track it and they were not going to let it go to waste.
I am guessing they watched you shoot and leave and figured you were like many others out there who don't follow up on their shots. They probably thought they were doing a good thing by tagging a "wasted" deer.