The moving body in Qyburn's lab in episode 4 of this season. Along with him saying he would "save" Gregor while Gregor was laying on the table then doing a bunch of medical-esque crap to the body wouldn't lead a show-watcher to connect these particular dots?
The show was pretty clear on Gregor being mortally wounded and Qyburn saving him. They have not been clear that Qyburn is skilled in alchemy/necromancy/black magic or whatever it is he's doing. But they did make it clear that he would "not be the same." I don't think this is too much of a stretch for the show watchers to figure out.
Since they have established necromancy with the wights (especially moreso with the last episode) it will be interesting if Qyburn's work achieves the same end result. An artificial Wight if that makes sense? Or if it is just some alchemical kind of flesh golem or something.