Passive infra- red
These sensors nowadays are ten a penny, i have used them very effectively in my properties, i have made homebrew alarm systems since the late 60's that were effective, now the availability of the small pir is available to anyone cheaply, the security installer claims he can step over a heat detecting pir i doubt it! also he said they are visible and can be avoided, not likely....the pir is very effective used as say security lighting they come on as someone approaches, using the pir internally will sound an alarm once entry has been breached the LDR ie light dependant resistor or PCC photo conductive cell had these step over properties the security installer mentioned, but were still an effective deterrent even though false alarms were frequent, the pir system is reliable and apart from leaving your pet bear in the room when you leave your premises unattended not much usually sets off the alarm except intruders.