Glenn's first and last times talking to Rick was through a talkie-walkie while calling him a 'dumbass', and both episodes ended with Rick being trapped in a vehicle surrounded by walkers. Well played AMC, well played...
Glenn may very well be dead but only back for flashbacks. I know some people have seen him on set with another upcoming major comics character but this may as well be for that closure thing they do to make you believe a character's not dead
If that's the case, they went out of their way for months. He, Steven, also reportedly filmed scenes with another current character in that same alley. All that aside, I really don't think he's dead and he'll be back and not in flashbacks.