No spurious false alarms have occured due to channel occupancy.
You have proof of that from the system's log of events ?
We are also into the area of signal strength are we not in other words the nearest sensors ie the ones in the local system are more likely to block an external system than an external signal is likely to block a local signal... Is that not true?
Good, you are starting to realise that reliable wireless communication requires more than just sticking off the shelf components in place.
You say the sensors in Mr A's system are more likely to block an external system. How does that affect Mr B whose system is external to Mr A's system.
Signal strength at the receiver is affected by distance to which the inverse square law, (twice the distance means the signal strength is one quarter ) but that is only one of the many factors that affect signal strength. The next thing to consider is the polar diagram of the receiver's aerial and how this affects the field strength required for the receiver to respond. Is the aerial omni-directional or is it more receptive to signals from one particular direction. Bits of metal close to the receiving aerial will alter the polar diagram,
Is the receiver AM or FM or PM ( phase modulation ) ?
AM receivers mix all incoming signals that are in the receiver's pass band while FM and PM tend to lock onto the modulation on the strongest signal. On AM broadacst radio you can hear two stations that on on the same frequency at the same time. The sound your hear is a mixture of both programs often with a whistle or whining sound that is the difference between the two carrier frequencies. On FM you normally hear only one or the other though the receiver may switch from one to the other as the relative signal strengths vary.