I agree, they didn't "just loose" them. Antlers are used for lots of differant things, from chandaliers, to calls, to parts of furniture, etc. To sell them would be very easy.
Hopefully you have a pic. or two or three of them. I would demend them back or threaten to bring in law enforcement, or like what was said about not paying the processing fee, as long as you got your meat, BUT that would depend on how badly you really wanted your antlers back!