[TimT]

Reality is that concrete cracks, not if...when..

The basement slab is not a structural element.

Was there any reinforcing steel or mesh in the slab? What sort of curing was done ? Or was the slab poured, finished, and no curing agent was applied (could be spray on membrane, plastic, or flooding with water)?

I had a similar thing happen at my house in Vermont... New extension and basement added to the house... a few months after the house was completed I noticed a crack in the basement slab..

Just monitor the crack, If you start getting water coming up through the crack you could saw and seal the crack.

I'd do nothing about it... just monitor it