wife's text on my phone

My wife and I don't share an AppleID, don't share an iCloud account, and haven't forwarded our messages. Somehow, while she was holding my phone (using speaker while on hold), a response to a text she sent from her phone 15 minutes earlier briefly popped up as an alert on my phone, and then disappeared. Her phone was in her purse on her arm and she was wearing her Apple watch on her wrist.

It hasn't happened since. How did this occur?