One of the many annoying realities of car ownership is the fact that a car's rearview mirror
is, by necessity, attached to the inner windshield with adhesive. The adhesive is typically a strong one -- it's usually easier to break the glass than to detach the mirror -- but heat and humidity can work together to weaken its bond with the glass. And there are few places that concentrate heat and humidity more than an enclosed car. Even in mild weather, this can break loose the rear view mirror.
If it happens that you're presented with a dangling mirror one day when you unlock your car, don't despair. It's just one of many small automotive irritations, but it can be dealt with -- and you don't have to pay a mechanic to fix it.