We have only our own perception of the world out there. When an individual is not looking at the mirror, or carries a conscious image of it, the mirror doesn't exist for that individual. That is why existentianalists accept that some things do not permit rational explanation. Unfortunately my dog does exist and the pong indicates a walk.
These types of questions have people going round and round in circles and coming up with the same conclusion. That there isn't a finite conclusion. What came first, the chicken or the egg?
that could apply to a lot of personal belongings other than mirrors, I can think of a lot of mirrors that are not mine that I am aware of in other peoples houses
But you don't know that mirror exists. they may have broken it 5 min ago and Surely nobody would phone everyone they had ever met to tell them a mirror has been broken.
I think that light and sound are nothing without someone to see it or hear it. If there are no eyes to pick up the light from the mirror, to what extent does light exist? We make these things exist.
So as far as I know the mirror does not exist as an image until I look at it. The universe has no reason to maintain an image without an observer present. I reckon it must 'warehouse' stuff until that stuff is looked at and quickly piece it all together.