Nothing usually happens until there is an inspection needed for some other reason or your local inspectors have nothing better to do than go around looking for trouble. You could end up having to pay for engineers to document what was done, pay for permits and have it all inspected, assuming someone wants to make an issue out of it. That would beg the question of whether the people who did the renovations could be asked/forced to reimburse you, where there is proof that they intentionally hid this situation from you (i.e., fraud).
In theory, if something "goes wrong" in a code violation, the person who put it there can be held liable, even 20 years later, since they caused the hazard and nobody else fixed it. Of course, there would need to be proof that there were no intervening elements that made it worse or that could/should have discovered it.
My fire department recently condemned a new furnace replacement (without permits, old guy down the road from us) and threatened to have the heating-company installer arrested because he failed and refused to upgrade the electrical and chimney issues. Those things had NEVER been previously "done to code" in that house. The new code clearly required him to "inspect" the chimney prior to doing the replacement of the furnace, and he certainly knew the emergency electrical switch was missing. It literally took 6 months to get it resolved to everyone's satisfaction.
So, in that case, the "last contractor to touch it" was held responsible to fix it, at his cost.