Couldn't за also mean 'inside', for example, if there is nothing interesting behind the window of a store? (i.e., you've already spent half of your shirt time in Moscow inside ГУМ, you have still not seen the Kremlin, but your partner doesn't want to leave, even though all shops are closed.)
It would be good to see a native speaker/moderator weigh in on this. It certainly seems like the most natural "primary" translation would be "There is nothing behind this window" as there doesn't seem to be anything indicating one is inside looking out as opposed to outside looking in. But then common usage may say otherwise.
I agree the literal translation "... behind this window" should be accepted, but perhaps DL should also accept the more natural English "... through this window". Maybe "outside" is also OK, though that assumes you are inside, so "inside" should also be accepted.
The point of this is not to have all translations... the goal should be that if the user understood the Russian and rendered it into acceptable and reasonably foreseeable English, they shouldn't then be tripped up.
i just learned that 'outside' is 'в улице'. To me this is another sentence i cannot translate. There is nothing outside ... you can not see throught the window ... There is nothing inside the box ... I could work with. But: there is nothing outside the window does not make sense to me