if I wanna be nitpicky, he only needs the senses to notice the carcass, afterwards he can examine it at any time. so Geralt seems less observant than usBecause it could be that you need the special senses to make sense of what happened to the carcass. They might not be apparent to the naked eye. The carcass wasn't the clue, what happened to it was. Forensics is afterall about the details.
it's probably done for consistency - they don't want the examine prompt floating above every single clue in the world, so they are always hidden by default. I would prefer if they weren't for bigger, more obvious clues, but whatever.. it's not that big of a deal.