Leaving Bluetooth on causes your device to keep broadcasting identifiers and scanning for nearby signals. Nearby actors can use that activity for tracking or reconnaissance. Most legitimate connection attempts still require a pairing prompt or approval on your screen, so being visible does not by itself give a stranger full access. Turn Bluetooth off in crowded places such as airports, cafes, or conferences when you are not using accessories. Set your device to non-discoverable or hidden so it does not advertise itself to strangers.