Hi Colin
I doubt any of the manufacturers would actually state that they dont transmit all the time anywhere on the paperwork. Between me and you, its sort of obvious this is how they work?. If they transmitted all the time they would be telling one and other to sound all the time.
Think about this logically if you want a light to come on in your house you have to switch a switch. The smoke detectors work the same way, if they want to tell the other smoke detector to sound they transmit a signal. If they worked the other way around it would be a waste of power, and if you had more than two detectors it would never work as the ones not triggered by smoke would still be constantly transmitting
So in basic terms: These type of detectors are activated either by smoke or by a wireless signal from another detector. For example if smoke is detected in the hallway, the smoke detector in the hallway sounds and at the same time tells all the other detectors in the house by transmitting a signal this tells the other detectors to sound.
All the smoke detectors are waiting to recieve a signal all the time, albeit wireless or smoke, but receiving is not the same as transmitting in the same way that listening is not the same as shouting
I hope this helps