I don't have an answer, so these are some ideas in a random order.
One difference I can see between a completely blocked sensor and one with no target would be the amount of ambient light. It's possible, I suppose, that in a room with no windows and only LED lighting, you would have a 'no ambient' situation, but generally you could use that. Might not work at night though.
Another thing would be to provide your own tiny target. Something just out of the FoV that would return a very faint signal. Something dull enough so it doesn't throw off your main target distance, but gets a few photons.
Perhaps you could turn off ECE every now and again. Just to check for the 'blocked' case?
Another thought... With a coverglass, a number of photons are reflected through the glass. If the glass were blocked, I'd expect a larger number of photons to be reflected. Could you -every now and again- turn off the ECE and get a measure of the crosstalk? I think that might do it. But it would take some testing.
Great question though.
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