What would they gain by looking at blue tooth connections?
"Hey , this guy's phone is connected to an ihome waterproof speaker"... that kind of thing?
Your comment may sound funny, but let's think what it can include:
First: Since it's a waterproof speaker, the guy seems willing to spend money to both aural media and audio electronics - a marketing option!
Second: His bathroom time can be measured, both for marketing purposes to make him enjoy his staying, and for criminal purposes such as burglary while he's enjoying an hour long podcast.
Third: A speaker can work as a microphone and the function of the connection can be changed using a piece of program code.
Third and a half: If he's connecting his phone into the speaker, there already is a microphone. It hasn't been proved yet, but numerous people have reported Facebook ads mirroring discussion near a laptop. Think about the guy being a shower tenor. If he seems to sing along "Bella Napoli" but not "Highway to Hell" with the ad loaded "free" Spotify, would the next ad spot include travel ads to Italy?