OK, I have no idea what the mystery signal is.
At first looking at your images I assumed it might be some kind of RFI, and probably local to you. It looks unstructured and incoherent.
However, I also receive it here (and on several remotes around the country), so it is not local to you, and appears to have some slight power behind it.
With having said I have no idea what it is, let me back pedal just a second. I have seen similar, but not identical, signals before. In the past I saw a spurious emission from a jammer that had a similar looking shape and drifting action. I don't know if that could be a candidate for the source of your signal, but if we could tie down start / stop times, and if they correlated to other signal start / stop times, that might be an indicator.
T!