I saw that my Heroes also recover from stress every week in town when they are doing nothing (no misssion and not in tavern or abby), but only very slow. How much do they recover in this way every week? Is this recovery a fixed number/percentage or random or does it depend on class, quirks/perks and actual stresslevel?
It is always 5 units of stress healed, a pitiful amount compared to the abbey/tavern or even stress healing while camping. Sometimes though - I'm unsure if it's because of an affliction or affect gained from questing or if it's just they are stagnant for too long - they will actually start to gain stress instead. They will notify you with a little something like "I'm getting restless!"
Sidenote: For stress I honestly just try to make my camping skills all stress related and have healers in the party (generally just self healers but that's up to you, I love how diverse you can make parties in this game and still be able to complete it).
EDIT: Wanted to confirm, here's a link: http://darkestdungeon.gamepedia.com/Stress