We received a card exactly like the one you describe. I came online to conduct research, see if I could ascertain the reason for the postcard and something about the sender. I'm with you: this has scam written all over it.
Called the number as they couldn't get my land line. Offered a free water inspection because someone in the neighborhood had reported a water issue. Told them I didn't need it and hung up. Pretty sure it is a scam as the city does free water inspection.