I've been having this exact issue for months.. Sometimes multiple times an hour or doesn't appear for a few weeks.. The pop-up is sometimes positioned relative to an open application but mostly it's pointing to a blank desktop. It pulls cursor focus away from whatever task I'm doing and makes my watch notify me to enter my password, all of which really irritate me! I've been trying to see what the console brings up with "passwords" & "Keychain" in the search field but no obvious culprits show up, there's mention of safari in the logs but nothing definitive.
"akd Error fetching keychain item - Error Domain=NSOSStatusErrorDomain Code=-25300 "no matching items found" UserInfo={numberOfErrorsDeep=0, NSDescription=no matching items found}" appears a lot of the time but I have a sneaky suspicion that it's more something to do with Safari, I have Brave as my default browser. Occasionally I use Safari and I feel this little pop-up is more "needy" around that time..