DamienB
Do mobiles actually transmit at all with no nodes nearby, and no call attempt actually being made?
Yes.
A mobile phone will search for a transmitter on all the frequencies that it is capable of and do so at maximum power. It will do this at an interval set by the manufacturer. This might be one minute or two, or even three but it will do so for as long as it is switched on.
In due course, combination units may have separate power switches for PDA type functions and phone but at present, as far as I know, they do not.
If I'm sounding 'preachy', it is due to 23 years in telecommunications and involvement with mobile radio for 19 of them!