As soon as you start the the level, head straight for the building with the elevator in it, to the immediate right of the second door way there should be a carcass. You need to do it right away or it dissapears for some reason that I can't remember. lol
Hope that helps you sir.
The Omega Strain, lol.
Yeah, this level in general was pretty glitchy, when I was going for the solo par last year trying to get all the pars since the online came back, I raged so hard at this map because of the glitches. Workers dying for no reason at all, workers' dumbass A.I. doing stupid crap and getting killed, falling through the map for no reason, the carcass disappearing for no reason randomly, etc.
Best bet is to do like he said and just go to the building on the right immediately and tag the carcass before it glitches up and disappears.