When I push my '08 990 around in the garage the front brakes make noise, a kind of hum or low pitch squeal.
Bike was recently washed and has only had a short ride since.

Is this normal or does it mean it's time for another caliper service?

I gave them a really good clean up at the start of the year (maybe 1,000 miles ago) but I'm thinking that wasn't a good enough clean, maybe the pistons now need to come out for a proper clean up?

Try freeing up the brake disc carrier bobbins first.

Just did that on my other bike but kind of thought that as the Brembo's are radial on the SD, thus are a fixed caliper with "floating" pistons, the disk would also be fixed and not floating?

Full/semi- floating afaik - bobbins should not be seized in position either way.

right, will give them a spin and a few blasts with brake cleaner.

if you have track stands rear + front, does the front tyre spin freely when off the ground or do the brakes drag it to a stop?

I do have a stand that can lift either end but I've put off doing this job until next year now.

I'm putting the bike into makeshift dry storage for the winter as my garage starts to get damp with condensation around this time of year. Often the bike can be literally dripping water!
Once I pull her out of storage I'll lift the front, clear the bobbins and if necessary clean the pistons.