I recently remembered an early Mythbusters episode where they tested the efficacy of common home security systems. They manage to defeat some motion detectors with a bed-sheet. But holding a bed-sheet in your hands only masks you from the front, so you need an all-around support to defeat multiple sensors.
Then I watched a video on the construction of a hoop-based Totoro costume which gave me the structure needed. This 'spacesuit' aims to reduce the thermal and sound signature of a burglar such that detectors and cameras are not triggered. It is ridiculous-looking but it might work...