From the Pocket guide (page 67) -
The real NAVFLIR hotspot detector has more features and options that cannot
be simulated on DCS. One of these features is sensibility. The real NAVFLIR
hotspot detector will detect ALL temperature differentials which creates
many false readings. Unfortunately, it is not possible to simulate such
sensibility in DCS, so the hotspot detector is limited to detection of active
vehicles (AI or player controlled vehicles). The hotspot detector in DCS will
not mark buildings or scenery objects.
NAVFLIR Hotspot Detector is not available on initial Early Access release. The
system will be enabled in subsequent updates.