Edit: Well I thought I had it conquered, but I don't. I just flipped it from bridged mode to not-bridge mode and manually disabled the wifi again and at first it seemed to have worked: I wasn't seeing anything emitting via my wifi analyzer app. And the corresponding LEDs weren't blinking either. But after about say 5 mins, it started acting up again.
Things are getting stranger. I noticed the following:
As I scan through other MAC addresses that I can see (I'm in a condo rental), I notice that the same thing seems to be happending for other MAC addresses being used in the apartments surrounding me. The primary MAC addresses which have SSIDs are also emitting on MACs with the same prefix (but different suffixes) and without SSIDs.
I'm hoping that by enabling and then disabling the public wifi via https://customer.xfinity.com/#/settings/security/hotspot that that will help me slay this issue. Somebody on a reddit thread seemed to have success with that.
It took me 36 hours before I finally saw the SSIDs (xfinitywifi) and now it looks like it will take another 36 hours before it's disabled. So just documenting what I'm seeing for the moment: it created two SSIDs (xfinitywifi), using the same MAC address prefix as for my private SSIDs. It used one of the what I'm calling a floating suffix: C1. And it seemed to create a new suffix to use: BA.
Turns out that while I bought the modem on ebay, Comcast had it in their database as being one of their own modems. And therefore wanted to charge me a $10/month leasing fee to use it. Which negated the benefit of me buying it off ebay. So I ended up returning it.
Never did solve the original problem. I suspect what was happening is that Comcast's systems were commandeering my modem thinking it was one of their own.