Mice are nocturnal creatures, so you might not see them during the day at all, meaning that you might only see the signs that they have been there. They leave dark coloured droppings around the place and constantly chew on things. Gnawed furniture, cereal boxes, or any number of other things, most importantly cables, are signs that you
have a mouse infestation. The chewing of cables, apart from causing serious damage to your electrical appliances, can lead to a fire. This is why it's particularly dangerous. The best way of noticing mice at night is to listen carefully for a scurrying sound.