This is a very good question, and obviously finding the answer is important. So we'll try to help a little bit.
It's hard to see how the steel could be magnetized much unless there were some magnetic field present, and the Earth's field is very weak. So maybe your guess about the microscope is right- perhaps it has a coil for some purpose, or holds something in place with a magnet. Why not take a little compass and put it through the usual routine of the instruments to see where it's in a strong field? Or (you probably have a big budget) you could be fancy and buy a Gaussmeter with a probe to stick in the places your instruments go to actually measure the magnetic field.
Are your instruments in a different environment than they used to be? Or have the instruments changed? Good stainless steel is very hard to magnetize.
Are they new instruments or old? If they're new ones, maybe you should sample something from another manufacturer. But I suspect they're old ones. Probably they're sterilized in an autoclave. The iron-chromium alloy may be subtly degrading under the high temperature, forming little pockets of easily magnetizable purer iron. If so, you may find that you just have to occasionally replace the instruments, since once this happens they will easily remagnetize even if you succeed in demagnetizing them.
So maybe the magnetism is a useful first indicator of metallurgical degradation. Even so it's odd that they're seeing a significant field.
Please let us know how this one turns out.
There might be some stray field from an MRI unit close by. The limit is 5 Gauss in publicly accessible areas, but that perhaps should be checked with a Gaussmeter. I agree with Mike that it's more plausible that it's a smaller magnet close by that's magnetizing your instruments.
(republished on 07/13/06)