Hate to say this but if you've seen one or two mice then you probably have a lot more.
We have a mouse problem, started in the chicken coop, then they went to the adjoining shed, the shed which is part of the house (the old coal hole) and into the greenhouse.
We have gradually cleared each shed, making sure the chicken feed is stored where they can't get it etc
Cleared the greenhouse at the weekend, the little buggers had tunnelled into my pots of overwintering begonias and eaten all the corms!
The dog found a nest and almost ate one of the babies, hubby dealt with the others, otherwise that would have been another 6 or 7 mice!
maybe you ought to think of traps if they are in the house, as the problem can so quickly spiral out of control!