> However, even in case the attacker is able to do so, the watch would still inform the user when the PC is unlocked. And the user can manually force a lock, from the watch, overriding the proximity/signal strength.

It's better than nothing, but the user is likely to think of it as a malfunction if they are far away (e.g. at a coffee shop), and the watch may not actually be physically on them at the time either.

And a second is really enough to plant malware on a computer; you can already buy a USB stick which types in commands much faster than a human: http://hakshop.myshopify.com/products/usb-rubber-ducky-delux...

Though that might be more of an argument about why this attack vector is unrealistic since most people don't even have full disk crypto on their phones/computers.

Also, not sure if you've seen this, but surprisingly these guys are still around: http://www.knocktounlock.com/