Biocube 14gal air bubble problem - I went 'Spock' on this

Quick background:

The wife hooked me up with a 14g BioCube for Christmas as something to tie me over while we work on our retirement home that will house a 180g in-wall reef. I have been keeping corals and managing retail aquatic shops on and off since 1995. Search my name on youtube to check out my older reef vids. Basically, this isn't my first rodeo.

So I have this 14g Biocube that has been set up sense Christmas and everything has been working just fine...brown algae, to green and so forth. All parameters are exactly where they need to be (tested every other night w/API Salt and Reef) as cured LR and LS were used. What I have done thus far is remove the bio balls, removed the plate above the bio balls, the water level is all the way up a few inches to the top in the chambers, I have kept the factory carbon plastic insert (which has been replaced once), I added the factory skimmer (which produces foam every three days or so) in the 2nd chamber, added a heater in the 2nd chamber just below the skimmer, in the third chamber is a pouch of nitrate sponge and on top of that is a pouch of always fresh Seachem activated carbon.

Now to the problem:

Just about every 10 seconds or so I will get a burst of fine misted bubbles shoot out the return into the DT. It isn't even close to being a steady flow of bubbles by any means...just every 10 secs or so. The bubbles are the same size of those produced by a wooden stone in a skimmer. Very fine.

What I have done to resolve this:

Turned the skimmer off, unplugged the power to the inflow to the tank so everything could equalize, used a flashlight to make sure there weren't any lingering air bubbles trapped at the surface of any of the chambers yet I am still getting air from somewhere. I can't see any air bubbles attached to anything, the skimmer is NOT the cause as I turned it off, air bubbles float so I eliminated air bubbles entering from the DT and I even made sure to squeeze any bubbles out of that dang sponge at the bottom of chamber 3 yet they continue to appear. So I am curious, where are they coming from? I am at a loss to figure this out but this is my first little nano so I might be overlooking something.

I titled this thread as 'I went Spock' as a joke but it reminded me of something he said during one of those movies..."if you remove the possible, whatever remains, however improvable, must be the truth"...

How is air getting from my DT, going through three chambers and then getting shot out back into the DT? It seems like I would have to have a leak somewhere for air bubbles to make their way to the bottom of the 3rd chamber in order to be shot back into the tank???

Confused. Any info would be great.

Thanks guys..and gals.