Multiscope Tool unexpected behaviour

When I use the multiscope tool in Frequency mode as an insert on an audio track, I expected to see frequency analysis of that track i.e high levels of a particular frequency showing a high display on the bar chart.
When I set the Frequency control to “Spectrum” that is what I see. However when I set it to 31 bands or less, the frequency display is showing a display no matter where the amplitude is.
This caused me much confusion earlier this evening when I was trying to pick out an awkward resonance. I realised the frequency display was still merrily dancing around on a “silent” passage on that audio track. I have edited my original post as I thought this was due to a signal from another track but after some further investigation I realised that the passage was not quite silent - there was a very low level of noise (but this was not enough to trigger any display when set to “Spectrum”)

This behaviour was unexpected to me - is it a bug or my misunderstanding?

System Cubase 6.0 on Vista 32.