The impetus for this question is that as of Rogue One, we have observed three (!) or even four (!!) instances of people or droids evading capture while out in the open in a desert with Imperial ships in orbit.
- R2-D2 and C3-PO on Tatooine (They are not found even though they are being searched for before they arrive at Mos Eisley. Then "These aren't the droids we're looking for.")
- Finn and Poe at Jakku (separately!)
- Jyn Erso on Lah'mu (where she evades capture by essentially running into a gully!)
All of which suggest the scanners are not capable of tracking known wide open targets at a known starting location -- a feat which is easily accomplished with today's optical satellite technology and was easily imaginable in 1977.
Yet in A New Hope, after R2-D2 and C-3PO eject from Leia's ship, we have
"An escape pod was jettisoned during the fighting, but no life forms were aboard."
which suggests at least, that it was intended that the scanners would have been capable of identifying people inside the craft. (Yes, there are other potential explanations, but this seems to be to have been the intent.)
So what gives?
What exactly are the capabilities and limitations of Imperial scanners?
(Related Did the sensors in Star Wars universe not have the ability to scan for droids?: That question is much more narrow and the given answers don't answer this question.)