Scope seals dry out with age and air leaks develop. As I mentioned before, submerging a scope into a tub or sink full of warm water will cause the air/gas in the scope to expand and leak out of any gaps. If a scope has been fogging up, there may be several streams of bubbles that come out. Removing the scope while air is still leaking out and drying it off quickly will prevent further moisture from getting in.
Scopes don't fog up unless moisture gets into them through an air leak. When a rifle is taken from a warm house or vehicle into rain, moisture accumulates on the scope surfaces. When the surface is wet and the air inside the scope (or nitrogen gas until it leaks out) shrinks in volume, it creates a vacuum, sucking moisture into the tube, and condensing on the lenses.
Even cheaper new scopes can have air leaks, so it makes sense to check them early and return them if leaks are found. Variable scopes sometimes leak around the power ring.