de0u Being a total wifi ignoramus, I would imagine that scanning doesn't involve any transmission at all, since SSIDs seem to be pinged at a constant interval. That's why using public SSIDs is allegedly more private, ironically, than using hidden ones: you don't need to shout out to them as you're walking down the street. On the other hand, some phones might do so anyway, which would connect the user's current location with the routers hosting those SSIDs back at home.
I know from a trustworthy source that there are low-flying planes which siphon up SSIDs and connect them to GPS coordinates. (Gasp! How could they ever have thought of such a shady line of business!) But of course Google Maps was sued for essentially the same behavior. And then there are all the phones around you which have their locations enabled and know where your SSID is hosted. So public SSID is bad. And hidden SSID is bad. Take your pick. Maybe a hidden SSID with autoreconnect disabled is the least leaky.