Getting someone else’s mail – who do I contact?

Last week we got a letter addressed to a random person (let’s call them ‘M’) to our house. Their name, my address. It was from Harris County Dept of Public Safety, and based on the look and feel appeared to be a drivers license. I marked it as return to sender and sent it back.

Today we received a voter registration card – their name, our address as the ‘send to’. However there’s another address on the card as well – their name, presumably their address – as the ‘registered address’.

Is this an identity theft thing? Who do I contact to sort this out?

EDIT: Dept of Public Safety said “can’t do anything for you. Just return to sender”. <eyeroll>