In the last couple days a RAF VOLMET (aviation weather advisory) broadcast has been heard with a very strong signal here in the US on 11159khz. The RAF did not change frequencies because the European signal is still on its usual frequency of 11235khz.
After a good bit of analysing of the signal by myself and others in the shortwave listening community, it appears as the signal is originating from somewhere in the Western part of the US. Another curiosity is that the broadcast is the same as the normal VOLMET broadcast on 11235khz but slightly out of sync with it.
As of this morning, 11 August 2013 @ 1602z, I am not receiving it at my station and there have been no reports from anyone else that it is still on the air. I will update if it is still broadcasting. [EDIT: Just received a report that it is still broadcasting.]
This whole thing is very odd. It's mysteries like this that make shortwave listening so interesting. Here is a clip of the audio I recorded of the mystery VOLMET broadcast.