Q: ultrasonic sensors - distance detection
Doesn't the sensor have a separated receiver?

As you said it could be that the received signal is highly attenuated and the SNR is so low you cant see it. One way to find out if there is a copy of the pulse inside the data is using a math tool called auto-correlation, but I doubt that a simple ultrasonic parking sensor uses it.

To have a better idea you can try to track the PCB and find out if you can see a filtered signal.