Most Bluetooth headsets are not secure. I encourage everyone to watch the video linked below to see how easy they are to hack.
In this demonstration by Joshua Wright he connects to a stranger’s bluetooth headset and is able to eavesdrop on the random stranger. He also briefly shows how audio can also be sent to the headset. Anyone with a Bluetooth headset that’s currently on is at risk of something like this. The biggest part of the risk is that almost all Bluetooth headsets use a default PIN (usually 0000).
Watch the video here.