I'm almost always opening 2.5 on button.

My primary consideration for fish being in the blinds is from EP/MP when I want to guarantee I isolate the fish and have position. In that case I might 3.5-4x from EP/MP, compared to my standard of 3x.

That being said I didn't notice that there were fish in the blinds when I made my post. "pretty much unknown but looking weak passive so far", sounds like not much info and prob not enough to deviate btn steal sizing. Even still my primary motivation for sizing it bigger is usually to isolate the fish and have position on them, and not necessarily for pot building purposes. In this case we're already guaranteed position.

Really though your steal sizing should be part of your overall gameplan. If you're only stealing 25-30% then sure 3x all day, your range is strong enough to do so. My btn steal is probably 55%+ unopened pot, so 3x would be overkill.