Look, if you watch the preview of episode 4, it clearly shows i was right that the father/writer created extra ' character creation ' to try & kill ' Kang Chul ' on the rooftop & with that certain knowledge, it was also the father/writer that created the other character in black, in the penthouse.
Him disappearing in the elevator is PROBABLY because it couldn't complete its purpose of being there, which was given to him by the father/writer, before his drawing device was destroyed that's why he PROBABLY disappeared after the drawing device was destroyed, since he was the father/writer's CREATION, & there was nothing drawn about it & can be after trying to kill ' Kang Chul '.
& the truck didn't vanish in thin air because the drawing device was still operational at the time, which i think gives the father/writer the ' power ' to make something happen in the ' W ' world, so after ' Kang Chul ' forcefully overcame that with his own ' power ' then the story continued on its own & the truck continued on it's own way but didn't disappear because the drawing device was still operational. if you get what i mean ?
Also in episode 4, we know by now that the drawing device is destroyed & along with that so did the father/writer's creation of the truck disappearing as well. what more can i say? xD
also the purpose of the nurse, which the father/writer gave her, trying to kill ' Kang Chul ', as well disappeared, that why she says that she doesn't know why she tried to kill him etc
but you have to remember that this is just my theory, which, i'm certain about so yea xD