The passive infra-red and microwave detection technologies would usually detect all movement of body heat and assesses changes in the thermal environment constantly. When a room is empty, the environment stays stable and there is unlikely to be any sudden changes in heat. When someone enters the room, the passive infra-red emitted registers as a big change in the environment. This triggers the alarm system. When set up for pets, the system is set to ignore heat signatures of a certain size and is often set at a certain height. This works well for pets under 45kg in size. This is also based on pets that don’t jump up on to surfaces or against doors and windows. This may require a more creative solution. There are also systems that cater for larger pets of 40kg+ but these are a little more specialised.
It is also recommended that you have cameras installed when you have pet friendly systems installed to allow you to view if the alarm is activated due to an actual break-in or disturbance or if you pet has found a way to set off the alarm despite the pet-friendly precautions.