How to listen quietly for other Bluetooth devices?

[Mikhail T.]

Hello!

I'd like my FreeBSD computer to detect the presence of other 
Bluetooth-devices nearby.

I have a little USB-dongle plugged into it, which is recognized as ubt0. 
I started "hcidump -a" while turning Bluetooth on the iPhone next to the 
dongle on and trying to link to a pair of BT-headphones. The "hcidump" 
didn't print any traffic even though, of course, the phone did try to 
radio out.

How can the detection be made to work? Note, I don't want to decode any 
communications between BT-devices. I just want to detect their presence 
and what information about them, that's not encrypted...

Thanks! Yours,

    -mi