Got something similar here in the UK also. I once had a Linux server box running on my DMZ, got a few physical letters from my residential ISP (Virgin Media UK) saying they detected some open port that was recommended to be closed (Think it was NetBIOS port).

Might have been part of this scheme.

Don't have that box anymore (was around 5 years ago) or a PC on the DMZ so haven't received any since.