How is music playing on another device when using a premium account?

[user-removed]

From time to time I will be playing music at the gym on an iPhone using a premium account.  All of a sudden someone at the house will play music at home and change the song on my iphone at the gym. We cannot figure out how or why it is happening. Any ideas?

Plan

Premium

Country 

United States

Device

Multiple 

Operating System

(iOS, Android Oreo, Windows 10,etc.)

[Wendy_Schippers]

Could it be it's an open WIFI they connect on from the Gym and then have the ability to stream over the Gym's speakers?

[user-removed]

This is a good possibility. She is an instructor and she connects to the amp using a cord. I will look into this further. Are there any other ideas out there?