Garmin’s literature states that a 1cm reading indicates a no return signal and a 5cm reading indicates an invalid measurement. I get neither but get a 0cm reading when there is no target in range or when it’s pointed at a reflective surface. Is the literature incorrect? I have asked them directly but they offer no support whatsoever.
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Hey @drbrodie,
Welcome to the RobotShop community!
Yes, that sounds about right.
If I remember correctly, 0 cm is also a return value that is received when there is no return signal (like 1 cm), but they have different meanings.
I think 1 cm means some signal is returned but too weak to be used whereas 0 cm indicates completely no return or something along those lines.
Either way, as you’ve notices in your own tests, you seem to get 0 cm from cases where no signal at all coming back is likely, so I’d treat it as such. Experimental data is also a valid way of understanding an object! 
I hope this helps.
Sincerely,