Humm,
It is so long ago I set it up on my iMac (OS X 10.7) and it has stayed the same through all the updates since.
The Mac User Account was migrated to my MacBook Pro and then I sorted the Keychain item so that the Serial Number issue did not block my Account on that computer.
So the Account had the iMessages ID in there and I was not requested it.
I am guessing it is something in Hard Drive/System/Library/LaunchAgents or LaunchDaemons
The name might include iMessage items, Messages items or even the Soagent items
however at the moment nothing seems to stand out for certain.
However entering an ID then Signing out will also stop it but it shouldn't have to be done that way.
10:00 pm Tuesday; May 1, 2018
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)