You already have another radio: the Geolocation stack in the phone itself. It can't tell you precisely what side of the door you are on /now/ but it can certainly be used to infer how long you've been near the house and thus, where you're likely to be.

So say if I've been GPS "home" for five minutes it assumes 'inside'. Then when a Bluetooth sense happens next it is extremely likely I'm going outside. Do not unlock.

Same three minutes without a BT sense pass, it's now nearly certain I'm "outside" still instead of going in and out. Coarse GPS event happens showing the phone is moving from the home geofence? DEFINITELY gone. Do unlock when the next BT sense happens.

Edit: I don't think I'm being clear enough on this telephone keyboard. I feel that with enough GPS logic and location history you should be able to infer when the user is likely home and thus disable auto-unlock. This all explodes when two locks are used, though... Or two authorized users... Or two egress points... Hrm. This is hard. :)