How many bubbles? How big are the bubbles?
I've got a SWG and I can see very tiny bubbles coming out of a return line when the SWG is running. This is because when SWGs are running, bubbles are created in the process (google SWG bubbles if you want to know more). Edit: I'm currently running my SWG at 100% output and a low pump speed. I really noticed the bubbles coming out of the return line today. But they were very small bubbles.
I've also had large constant bubbles from returns. It started after I cleaned out my basket, so I concluded that I didn't seal the cover properly. After failing to stop the bubbles, I replaced the gasket. That didn't work and after searching in vain for a leak, I called a pool repairman who replaced a plastic part around the basket cover that I had cracked when I emptied the basket.
If you've got a SWG and they're tiny bubbles of no consequence, they're likely from the SWG. If they're large enough to be bothersome, you need to look for a leak. If you check your basket lid and don't find anything, either look further or it's time to call in a repairman. I would have never found the crack part so it was well worth paying the repairman to fix my leak.