At the driver level it's totally possible to do a purely passive scan that would be nearly impossible to detect, it's just not very practical so most OSes don't even allow you to force your radio to do that.

I think there's s some theoretical possibility that you could "see" the absorption of the RF energy in the antenna of a purely passive device but I think that would be extremely hard unless you're in an RF shielded box.

I'd answer the question "it can be purely passive but it's not usually done that way", which the top SU answer also states.