Seeing one mouse in your home usually means that there are others nearby. Having mice in your home is not something that should be taken lightly due to the risks that they pose to humans and their homes, such as:
Contaminating food and surfaces through their saliva, urine and fecal matter
Spreading harmful diseases and bacteria such as salmonella and hantavirus through biting, fleas, mites and their fecal matter
Damaging structural components of your home and your belongings from their need to constantly gnaw on hard objects