Qavion
We observed this effect some years ago experimenting with 173 and 433Mhz in and around metal buildings. These had a door and windows on one side only. When the transmitter was inside and the signal strength was measured on the outside of the opposite side it was quite strong close (~ one wavelength) weakened as the distance increased and then increased (presumably due to reflections from distant buildings). This was thought to be either:
The signal going out the door/windows and around the outside as a ground (wall?) type of propagation or
The building acting as a faraday cage/antenna
The one conclusion we reached in the playing around was that the higher frequency gave better results at the same power levels presumably as the shorter wavelength went through doors and windows better. Any other conclusions were obscured by the fact that it was extremely difficult to get any form of reproducable results - then we started testing with a body lying on the transmitter.