I agree with the "it varies from congregation to congregation" comments.
When I stopped attending, I received one courtesy call from the presiding overseer who made it clear he had no intention of prying. It was evident he was simply going through the motions and checking a box. I don't think we wanted to be burdened with the additional work that would result from discovering any "wrongdoing" on my part and I can't blame him.
Contrast that with an elder at a neighboring hall who acted like a private detective. He was known to conduct stakeouts to smoke out evil fornicators, a real prick of a human being.
I've lost touch with what's currently going on in that world, but disfellowshippings were still quite popular in my area as recently as the early '00s. It was hard to fornicate in secret within such a small, tight-knit group.