There are ants that nest in houseplants; I am familiar with a small black unaggressive species that does that, sometimes a huge population developing in a big pot. The larvae are white and in scale with the ants. All you need to do is watch to see if ants are coming and going, and if so you can treat the outside of the pot and surrounding surface with a residual ant spray, which would eventually wipe out any remaining colony.

They dig out cells in the soil and somehow waterproof the roof and sides.

Machaelg
You described it. That's what i had. The pocket was cleaned out but still saw some ants and larvea so an ant trap was set on top of the soil.
It should work. My fingrts are crossed.
Paul