What is going on with Carol???
I will use spoiler tags only because some of it draws on a similar element from the comics, but this is really just speculation at this point.
Something weird is going on with Carol. Is she dead or alive? I think she is alive, but not in a good place. I think Carol is alive and being held captive somewhere in the prison. I think the person who has captured her is Axel. Here is what I think happened...
Carol goes through the door and leaves T-Dog. Shortly thereafter, she runs into Axel and she asks him for help. She leads him back to where T-Dog was and once her back is turned, he knocks her out and drags her to a cell or ties her up somewhere. He then took her do-rag and left it near the walkers eating T-Dog so that the other survivors would assume she got eaten too. Now he has a captive/sex slave whom he can use whenever he wants.
Rationale:
1.) Axel seems to be the tv version of the accountant/nerd guy from the comics who was a sexual deviant.
2.) Axel is also someone whom we do not have a complete knowledge of his whereabouts during last week's episode. Rick, Daryl and the black prisoner go to shut off the generator and run into Andrew. Glenn and Axel go off to search another area of the prison. Glenn and Axel could have separated for a short time during which Axel runs into Carol and captures her.
3.) Axel seems too nice and harmless, so it will be a big shock to the viewers.
4.) Carol closed the door behind her when she left T-Dog and she had her do-rag on. When Rick and Daryl find T-Dog, they also find Carol's do-rag and the door is open. Someone went through it and left it open. It could have been Carol or it could have been Axel making it look like Carol did by leaving her do-rag as a red herring.
5.) This season's tag line, "Fight the Dead. Fear the Living."
If this isn't what happened, then I'm going to apply to be a writer on the show. ;-)
Spoiler: 
