There are two big differences between passersby and unobtrusive fixed cameras.
• Passersby are generally easily noticeable.
• Passersby pass by.
A level of care that would make one safe with a high probability against being seen by passersby might fail completely against unobtrusive fixed cameras, so I'm not sure that the passersby-could-see standard should be the one used for cameras.
For example at a house my family had when I was a kid we were out in the country in a flat valley. From the end of our driveway I could see far enough up and down the road to see any potential passersby a minute away and postpone anything sensitive I wanted to do in the front yard until they had passed by.
On the other hand, I suppose that even if we decide not to allow law enforcement to use unobtrusive fixed cameras in situations like this it won't stop private parties from using them. Any private property that has a view into your private property could be a place where someone has decided to do some video voyeurism and is recoding.
Maybe the technological cat is out of the bag and we are just going to have to accept that if you don't want something seen you need to do it somewhere where there is no visibility from anywhere outside your property.