What ASA u are shooting at that moment, it is at ASA 400 and above, it can be noise in higher mode when shooting night scene, the compact CCD is small compare with the SLR, that's why u got white dots

Is it everytime u should at night scene the white dot appeared at the same spot, if is the same then I recomend u to send to Canon to clean the CCD, if it is stilll under warrenty, the service is free, sometime they can clean it for u on the spot, don't try to clean your CCD yourself, is very sensitive, if u scratch it is consider as gone

Try this, shoot some pictures in various ASA on the sky, if there is blue sky then it will be much easier, then download your pictures in your computer, view the images at 100%. If you can see the spot, then definately is the dust at your CCD

This is from my experience