So I found a very small colony of these ants inside an old, dry fence. Very interesting choice of nest! They were nesting inside the cracks of the wood, and I couldn't see any shavings anywhere. At first, I thought these guys were tetramorium immigrans, because of their shape and size. But then I noticed that they all had red abdomens instead of black. And also t. immigrans doesn't typically nest in wood, so maybe this is a different species? Here's some pictures.
This being compared with an ordinary t. immigrans, pictured below: