All cameras work differently. It depends on the flash, the lens, the shutter speed, even the angle of the dust. This looks like a piece of dust that is really close to the camera. Depth is nearly impossible to determine when you are dealing with reflecting dust. As for why there is not any other in the frame: maybe there was not any other dust at the time, maybe this is the only one at the right angle or material to reflect in to your camera. There are many reasons why there are not other ones in the frame. But it looks exactly like dust that i catch all the time in my investigations.
Orbs are pure energy and they can be seen by the human eye. Did you see this with your own eyes?
Do a little experiment. Go in a dark room with your camera. Bring a pillow or a cushion in there and start banging in around to release the dust and start taking pictures. You should have a lot of these in your pictures.
