Biometric passports: proximity for it to be read?
 

How close does your passport need to be before passport control/similar locations are able to read/pick up your passport?

Just curious :)

It is passive RFID, so a few feet at best.

Aha, ok. So if you have other passports with you at the time of passport control, I guess they'd be picked up?

Do any countries ever get huffy at this, from a multiple citizenship holder perspective? I know some do with their own citizens, but I mean somebody going to a purely 'third' country not related to any of his/her citizenships.

No, I doubt any country is actively monitoring for other passports or trying to detect passports at that range.

Even if they were, if it is in a bag it would be difficult to detect.

If you are paranoid about having other passports detected you can wrap the ones not needed to be use in aluminum foil.

Biometric passports
 

I always travel with two passports, both with biometric chips.

The second one stays in my pocket but I haven't run into any issues over the last 5 years

Or use the foil to make a decorative hat.