They're passive.
https://roll20.net/compendium/dnd5e/Ability%20Scores#toc_8Edit: Okay, I see your point - wouldn't require a roll if it were 5E passive. Still, I'm not searching; if I walk by and the check occurs without my intervention, it's passive. Tabletop simply does the set value to not alert players (which BG3 does when you do a perception check anyway).