Air tags do strange things. My air tags on my work items like my briefcase, lunch bag and id badge are not detected by my daughter when we drive to work, but when we walk home together, her phone warned her that an AirTag was following her.

In your case, if your husband took his phone with him, the AirTag separated from his phone. He would have gotten a left behind message and you got the air tag nearby, which is supposed to happen.