Using srf05 ultrasonic sensor to detect changes only
The SRF05 sensor is designed to give a pulse indicative of the time it took to receive the first reflection of the US signal. Hence when a nearby object is present a more distant object will have no effect at all on the 'official' output.

Somewhere on the PCB you can tap into the raw received signal. You could process this yourself. If the more distant object produces a US reflection it will be observable in this signal.