Weird behaviour of K-m dust alert - any ideas?

My K-m has started doing some odd things with the dust alert function:

If I check with a zoom set to 50mm or wider it comes up perfectly clean.

If I check with the same lenses set to 70mm/80mm I see horrible marks everywhere (which seem to be in different places every time I try). These are my F 35-70mm and FA 28-80mm W/Power Zoom.

If I check with my 100-300mm zoom set at 300mm it comes up clean!

Any ideas? I'm wondering if it's dust inside the lenses as for obvious reasons I doubt there is dust on the sensor - it comes up clean with the kit lens and the big zoom. I initially wondered if it was dust that was only showing up with a higher magnification lens, but as it comes back clean at 300mm I doubt this is the problem.

Dust in/on the lenses has almost no detectable effect on the image unless it's chronic, in which case it just blurs it. It's nothing like dust on the sensor.

You can get erronous Dust Alert results when the lens is capturing anything other than a plain image. A different focal length may capture something different that confuses the Alert system.

Try testing manually in the old fashioned way, like this:--

Select Av mode and f32, ISO 100, exposure compensation +1.

With any lens, take a shot of a sheet of white paper. A hand-held long exposure is OK. Make sure the whole image is white and you're not going off the edge of the sheet.

Check the image at high magnification for spots. Don't be too worried about small or feint ones. A sensor is rarely perfectly clean. Don't confuse digital noise for dirt either.

I've been trying the plainest subjects I can find - a whiteboard and a grey sky. I do suspect that it's related to that as I once got a large black mark down one side of the test shot which was roughly the same shape as the edge of the target (a piece of white paper which wasn't completely flat). The rest of that test shot was completely clean.

I have had problems with dust alert showing up dust on the rear element of lenses before now, so I'll check that those are perfectly clean too. I know it's possible to heal it in the GIMP (and I've done so before now) but obviously that's a pain to have to do all the time.