robtillaart:
So the question is, who or what sends it?
It's one of these: Electricity Power Cost Monitor.
The module has a signal strength indicator, so you can use it to detect the sender? (neighbour, TV, elektricity meter, washing machine ...?)
Sort of. The received signal strength peaks every ~31.8s, which coincides with the Power Cost Monitor's display updating. I get signal strength traces like this: http://scruss.com/wordpress/wp-content/uploads/2011/02/signal_strength.pdf
(If that image is blocked, there more of a blog post here: on the trail of the elusive Power Cost Monitor signal)
cheers,
Stewart