Using a 9V battery as dummy DC source (source doesn't matter here )
My 1054Z shows a clean 200mV peak to peak signal at 100MHz,
It's AC coupled, timebase is 5ns. The factory probes are definitely set to 10x.
The scope is 50MHz unhacked - so I'm somwhat surprised it even shows a clean signal.
I've tried swapping probes, and swapping channel inputs with the same result. Input is hiZ (there's no physical 50ohm termination).
Turning on the 20MHz BW filter and the signal drops to 70-80mV p2p.
The only thing I can of is a couple of local FM radio transmitters in this area - 100.9 and 99.3. Would that make sense?