Literally same thing happened to me, back at the end of 2016– turned in my modem to Charter, asked if I was all good and making sure my account was closed because we were going to be out of the apartment, all seemed good. Then in early 2018, I started using Credit Karma to track my credit score and make it go from awful to not awful, and lo and behold I have something in collections from one Charter Spectrum.

I hope all the big Internet monopolies get a rude but humble wake up call someday soon.