Bluetooth is a very common feature on mobile phones today. Unfortunately, some implementations have flaws, making it possible for an unauthorized device to connect to your phone, such as shown in this video:



Short answer: turn off your bluetooth if you're not using it. Don't unnecessarily make yourself a target. If your bluetooth is disabled, they can't get in, simple as that. Personally, it's why I tend to use a wired headset. That and the sound quality is just so much better on a wired headset.