Lucifer refers to God as having: "..went out for a pack of smokes and never came back". Lucifer actually killed Castiel in 'Swan Song'. Nobody has a good answer, that I know of, as to how he keeps being resurrected. The brothers seem to just accept it. We all assume God is there somewhere, but just staying hidden. But Lucifer should know better or think something is off, knowing that he blew Cas to bits. When Cas shows up with Dean at the cage. Lucifer messes with Cas by acting like he doesn't remember Castiel's name. But he knows it is Cas! The same Cas he killed and unless God really is back from taking a walkabout, Cas should not be there! Seems like this is something that has gotten lost to the writers.
Has anybody heard of any discussions on this? Or have a explanation?
Okay, this is a hard one to search on the internet. But found something. Not a good something, but plausible: Death
God had bound Death, who was later summoned by Lucifer to work for him in the Apocalypse, starting with Chicago. Dean in his attempt to kill Death, brings the Scythe back to Death. The latter doesn't want to be controlled by Lucifer and wants "The leash around my neck off". Later with the leash off and Lucifer back in his cell. Perhaps Lucifer knows that Death would have a grudge against him. He did after all have the power to enter the Cage and retrieve Sam's soul and leave with it. Lucifer would also know of Deaths power.
So it could be possible that maybe Lucifer just thinks Castiel's appearance is nothing more than Death resurrecting him. Death was killed in Season 10 Ep 23. The last time Castiel dies is Season 9 Ep 03. Even if Lucifer knows of Death's 'death' he would have still been around for each Cas Death.
I can see the possibility of Lucifer thinking Death might have been getting back at him after Swan Song and resurrecting Cas. Perhaps he doesn't know about the other times. Not that I think it is Death doing this, even though he was still around during each time Cas died. I'm just suggesting it for a possible reason why Lucifer might not make any reference to Cas being around. Only problem with this is that Death likes the natural order of things. He even states he don't normally bring people back even once, let alone twice.
Another possibility is that maybe Lucifer entertains the idea that it is just something God set in motion before he took a walkabout.
I still think it is something that writers have forgotten to cover. Even if it is found that God is doing it, they still need to explain why Lucifer didn't notice.
Found a funny video with Death in it on YouTube that has nothing to do with this called :
Starring our favorite Death - Julian Richings