Loosing Volume During the Long, Slow Fermentation of a Sour?

[TAK]

I have thee sour projects underway. Each, I brewed enough to fill a "6.5 gal" carboy all the way to the neck, which is 7 gal.

Of the three projects I have, one has stopped producing CO2, and the others are slowly but continuously showing bubbles rising through them. Of those two, one appears to be losing volume. See the pic below.

It's not much, maybe a couple ounces. But only one carboy is doing this. The others, not at all. In the pick, you can see the dark ring a bit above the liquid. That is where the beer used to be. It was at that level for monthes, and then just seemed to drop. The carboy is bunged up well with an airlock that I've always kept topped off.

Anyone else experience this small volume loss in a nearly airtight environment, or have any insight to way the beer level would drop like this?

 

[TAK]

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[WI_Wino]

I see volume changes in larger topped off carboys as the temperature changes. Warm to cool isn't too bad as the level goes down but be careful as a carboy warms up. I've had wine expand up through the airlock and make a mess.

 

[TAK]

I didn't consider thermal contraction. I suppose that's the most likely candidate. Odd, though, that the other two carboys didn't exhibit the same change.