Usually, when a member of the cast dies, there is someone there to witness it.
The only other time a significant character died when no one was watching was back in Season 2 with Sophia. With the cast not knowing the whereabouts of the missing Sophia, who turned out to be a zombie in the finale, they spent the rest of the season in search of her. Will they do the same for Glenn?
No one saw Glenn die. The last to come in contact with him were Michonne and that Dr. Dre guy from Straight Outta Compton. If Glenn were missing, he would certainly not be presumed dead. The others will go out looking for him, especially wife Maggie.
If the zombie horde did get to Glenn, they would have eaten every last morsel of his pizza-filled body, leaving Maggie searching for Glenn forever, and without closure. Don't think the writers would leave that big of a loophole in the story.
Until then, The Walking Dead has once again ripped our hearts out of our chests and it was magnificent.
The Walking Dead will return this Sunday with new episode.