Humans invisible to heat sensors?

["Roselnathan" <rosehallz@xxxxxxxxxxx>]

Anyone heard of clothing or other device which makes an intruder
invisible to heat-sensing type of sensor (part of domestic security
sytem)?  (1) Last night friends activated ground floor zone at 11.45pm.
(2) Approx 2.55am burglars threw paving stone through ground floor
window, got in and left with 42" HD plasma TV - alarm did not go off.
(3)  Owners woken by noise, went downstairs, switched off system at
3am.  Today alarm professionally tested and apparently working OK.
Print-out shows 1 and 3, no activity of any kind between.  What
happened?  Vague hint that there are ?clothes ?devices that make the
wearer/user invisible to these sensors.  About to take up expensive
contract with national name (UK) and would appreciate advice.