I don't think you need anything as expensive as an XBee, do you?
A cheap 418MHz (or similar) Tx/Rx should do the trick (watch out for false triggers from door bells!)
Some of the Devantech serial/I2C ultrasonic devices allow a "listen-only" mode, so don't require any surgery.
A problem I think you may encounter is the relatively narrow acceptance angle of some ultrasound receivers - off-axis reception may not be very sensitive.