Mark, ALL radio signals are vulnerable to being intercepted. All. Period. I was in the business of trying to install a secure network for the US Government in 1987 (33 years ago). As part of that process, we had to show that all of the devices were safe from being listened to by anyone. In a laboratory, I personally witnessed the exposure of a supposedly "safe" computer to a computer in an adjoining room. Every keystroke, every mouse move, every character on the screen and every byte read or written on the drive was visible. And that was on a PC that was supposedly "hardened" to protect it from just what we did. The signal was leaking through the joints in the case, through the connectors for keyboard, mouse, monitor. It leaked like a sieve. And it had been "hardened" at fabulous expense to be safe! At that point I came to realize that there is NO SUCH THING as computer security. And nothing I have seen in the 33 years following have made me think anything is different today. As long as your computer doesn't radiate so much that it interferes with your other devices, or that it becomes a health hazard, nobody cares about how large the signals from it are.
So, "can" BT be hacked? Yes. But that is the wrong question. The better question is "How likely am I to be hacked over BT?" Unless you are a foreign agent who thinks the government may be tracking you, or a master criminal who has the FBI on your case, or something like that, the likelihood of you personally getting hacked over BT is miniscule. If you are in a single family dwelling, I would expect that unless your neighbor has very fancy equipment to surveil your house, your BT signals are pretty much invisible at that distance. Now, in a dense apartment complex, your neighbors on either side, up and down, can probably see your BT devices when they are in discovery mode and advertising their presence, along with your WiFi devices, but again, unless you are in one of the categories I listed, your risk is relatively low. About all they might get on your keyboard with very expensive equipment to surveil them are your passwords, but even so, they probably won't know what account those passwords to go. They cannot see where on your screen you click the mouse, or what websites you click on to visit, so even if they get your login and password, they don't know what accounts those work with. Frankly when you transmit those same logins and passwords over WiFi you are at greater risk of exposure than in the interface from the keyboard or mouse to the Mac.
Bottom line: The risk is small but not zero. Probably too small to worry about, unless you are a foreign agent, master criminal or something similar. But in those cases you wouldn't be searching for security answers on a public forum (or shouldn't be, anyway).
