They did something similar to me too. I dropped off the modem, with my two small kids on hand, and forgot to grab a receipt. I figured dropping it off in person would leave no room for doubt. That was not correct.

A few months later, I'm getting threatening calls that I need to return the modem. I call them and the rep says "Oh, thanks, we will note that you returned it." So, they just send a collection notice and don't actually keep a record you have returned it inside their own systems.