Getting Apple authentication notices. What’s causing this?

I’ve started getting messages on my iOS devices that something is accessing my Apple account, providing me with a “allow access” and a 6 digit confirmation code. The thing is, I don’t know what’s requesting this. I’m fairly certain (99%) that it’s HA related as it happens during restarts. How do I discover the culprit, and where do I enter the 6 digit code?