what finish is the slide? nickel, blued, etc??
Something had to have come into contact with it- holster wear, chemicals, sweat, high humidity, etc.. something of the sort. It would be exceedingly rare to see a nickel slide develop rust spots. A blued slide, sure, no reason why not, especially if there isnt a layer of protection on the slide such an oil barrier.
but for a blued slide to just randomly develop rust spots with no scratches, abrasions, scuffs, or liquid contact, I assume could only be caused by excessive moisture in the air and constant exposure.
Bersa pistols don't all have fantastic finish quality, but they certainly aren't poor or bad by any means. Can you post some pictures?