GAW
chas vesholom!
There is no such thing as Sanhedrin killing without eidim.
The Gemara (in chulin i think) that Sam alludes to is discussing doing an autopsy on a murder victim (where the cause of death is known there are eidim etc…) in an attempt to prevent the murderer from misah, since if the victim was a treifa then there is no misah for the murderer.
There is a Tosfos in Bava basra that Allows Autopsy in specif situations for finacical gain though I forgot the specifics of hand. (The Gemara discusses a fight over wheterh a seller who died was a gadol or KAtan making the sale valin or invalid respectibly nad R’ Akiva (?) offers two answers why autopsy isnt allowed 1) nivul hames and 2) it wouldnt be conclusive. Tossfos says both reason are neccesary since Nivul hames alone wouldnt allways prevent autopsy if there was a (financial) benefit. Though again I dont rmeber the specific application of Tosfos