Bathroom mirror fogging up even when bathtub is not in use
If it's fogging up without a local source of high humidity there are a few other things that could cause it (probably in combination):


The mirror being mounted on a cold/badly insulated outside wall. 
An extractor fan in the bathroom drawing in steam from other sources (cooking, another bathroom etc.) which should be ventilated locally. 
Damp, for example from leaking plumbing or weather getting in 
Drying washing or towels over a heater in there