US Patent 7466224 System for detecting intruders in a populated space

A system for monitoring the presence of persons within a zone, the system comprising: one or more remotely detectable markers, each capable of being carried by a person; a first sensor arrangement capable of identifying the presence of the body of at least one person within the zone; a second sensor arrangement capable of identifying the presence of at least one marker within the zone; and a processing arrangement responsive to the first sensor arrangement and the second sensor arrangement to generate an alarm signal if the first sensor arrangement identifies the presence of a body without the second sensor arrangement identifying a marker corresponding in a first predetermined manner to that body.