Air bubbles from one return only

[Paula]

There are air bubbles coming out of one of my pool's return outlets. The other outlets produce no air bubbles.
There's obviously an air leak but why doesn't it show up with air bubbles coming out of the other returns?
I don't seem to be losing much water, if any.
Where do you think the hole is?[/code][/quote][/u][/i][/b]