Why does Mac ask me to Sign in with Apple ID on every login?

Hi! Every time I restart my MacBook Air, it ask me to Sign in with an Apple ID to continue using account services. The odd thing is that I am already signed in with my primary Apple ID. The Apple ID it ask me to sign in to actually belongs to my uncle! He has not used my computer or signed in to it with his Apple ID. I sometimes help him out with his devices and I might have signed in to his iCloud on Safari through my MacBook. But I am not syncing his emails, notes, calendars, photos, or anything like that. So, it's a bit strange that my computer asks for his Apple ID password. I cannot figure out which particular service(s) on my computer require his ID. I don't remember his password either, so I just cancel the popup every time. Any help would be appreciated!