[Plimsoll]

While I'm not archaeologist that sounds about right in very theoretical level. To explain this more practically it comes down to heating properties of matter.

As specific heat capacity is inverse proportional to density, sand and pyramids react to temperature changes differently. In desert temperature changes between day and night are huge causing sand and pyramids to have different temperatures. As thermal radiation is infrared in temperatures near human body temperatures (didn't check the exact temperatures but pyramids should fit in infrared spectrum), sand and pyramids will show up in different 'colors' in infrared mapping of area.

This is probably the basic idea behind these findings, of course what they are doing certainly goes beyond this level of explanation with use of infrared spectroscopy.