Pop-up stating "Your Apple ID and phone number are now being used for iMessage on a new iPad."

[fwiw, I've seen similar posts, but I haven't found one that exactly matches my issue]

Just now during a call I got a system popup while I was on the home screen (springboard), stating that "Your Apple ID and phone number are now being used for iMessage on a new iPad". It states "If you recently signed into "my device name" you can ignore this notification." The iPad name shown in the popup is correct. I've seen this message once before in the past few weeks.

Here's the problem, I didn't sign in on that iPad, and when I unlock it and go to the messages app it asks me to sign in, so I clearly did NOT recently sign into iMessage on my iPad. Looking on icloud.com I see that only three devices are signed in: my phone, my watch, and my iPad. Seems fine.

My question: how could I be getting this prompt if I didn't log in myself? Is this a known glitch that may occur days or weeks after a password reset, or is something funny going on?

I'm extra concerned because it seems a few weeks back someone tried to log into my account. In that case I got a system popup "Use this phone to reset your Apple ID password (allow / don't allow)". That's very vague for an official popup, but that's another story.. Based on advice from macrumors I reset my iCloud password at that time, so maybe that's why I'm getting iMessage weirdness, BUT: I did not log into iMessage on the iPad, so I'm still confused why I would be getting the popup described at the top.

Ideas anyone?