loj is used for something that incrementally or gradually (not necessarily slowly) goes away. An example might be food on your plate or dirt that you wash off of something. If the room the person walks into is filled with, say, boxes that are to be taken somewhere and then, over a short or long time, the boxes are removed, once the room has no boxes, you could use loj. If the person walked into the room and it was full of furniture, then the person left the room and came back later only to find the room empty (and there was no reason to expect this to happen), you might use ngab.

For beaming away, DemchoH might make sense for describing what people in the room who are watching someone beam away are seeing. That is, it's a way of describing what beaming looks like to those not beaming. But, of course, when beaming happens, the person who beams away doesn't stay there and become invisible. They're gone! Once the person is gone, you might use ngab. loj would be appropriate if you're focusing on the incremental disappearance of the person.