In the season 6 episode of The Walking Dead "Thank You" Glenn is seen as possibly dying. Many fans think Glenn is still alive for a few reasons. Glenn has gotten out of worse situations before, no main character was around to see him die, and he wasn't on the talking dead. However Glenn could possibly be a walker. This isn't the first time a main character had been killed when no one was around.
Merle is killed while no one is around...
In the Season 3 episode "This Sorrowful Life" Merle decides to try and redeem himself for what he has done with the governor and trying to hand Michonne over to him. This results in Merle killing many over the governor's men and almost killing the Governor. Merle and the governor end up getting into a fight and the governor then kills Merle and leaves him to turn into a walker.
Daryl looking for Merle finds him and is forced to kill his brother that he spent 3 seasons searching for. Yes Daryl left earlier in the episode to go find Merle but no one was around to see him die. Daryl after being attacked by the one armed walker Merle is forced to kill him.
In the episode "Now" we just found out the Maggie is pregnant with Glenn's child. This could possibly mean that Glenn is now a walker and in a future episode (probably the mid-season finale) will find him as a walker. With Maggie finding Glenn as a walker this will probably be the same scenario as when Daryl found Merle. This also could possibly put Maggie over the edge. Maggie has already lost a ton of people including her father Hershel and her sister Beth. This could even possibly lead to Maggie possibly killing herself because she feels she has lost everyone who is close and who she cares about. Me personally I am not that much of a Glenn fan but since he is one of few original characters left i would hate to see him go.
So this could possibly be what they have done with Glenn. When Glenn fell there is a slim chance of him surviving (yes he has done it before) but there wasn't really anywhere for him to go. Glenn will probably come back at the mid-season finale with Rick and Maggie finding Glenn as a walker and Maggie loses it but says that she has to be the one to kill zombiefid Glenn. Then in the second half of the season Maggie will probably lose it just like how Rick did and Maggie could possibly kill herself from the lose of Glenn and the rest of her family.