And here we see, via a simple thought experiment, why sentience is necessary (and probably a good degree of intellectual sophistication) for the perceiver to be able to quantum lock an angel. There is literally no scene where we’ve seen the angels on Earth that wouldn’t be crawling with other living things, most notably invertebrates (let’s not worry ourselves with the ubiquity of microbes). Wester Drumlins would have been home (inside and out) to hordes of inverts thst would be active day and night. In this sense, Angels on Earth would always be subject to the visual senses of many creatures (even if we don’t see said invertebrates via the camera)… yet this clearly doesn’t prevent the angels from moving. It isn’t enough to be seen, rather to be perceived by a sentient creature with a sufficient measure of understanding.
Got to say… I’m loving this topic. 
