In Hindi, पीछे देखो can mean both "Look back" and "Look behind (something/someone)". अपने (meaning: of oneself) is used to specifically imply the first meaning.
A: पीछे देखो। (Look back/behind of...)
B: किसके? (Of what?)
A: अपने! (Of yourself!)
The ambiguity above disappears if A says "अपने पीछे देखो।" in the first place.
The example was just to show the function of अपने. In normal conversations "पीछे देखो" is just as commonly used to mean "Look back" as "अपने पीछे देखो" because context often eliminates the need of अपने.
Is this "apne" the same apne we met before, for example in "वे अपने नाना के साथ रहते हैं" (they live with THEIR OWN grandfather)? that means that if I want to say "I look behind myself" I must use "apnaa" instead of the plural form "apne", for example "मैं अपना पीछे देखता हूँ"? Pls, tons of lingots if you answer
It's not a post-position, but as they often do it/the context does make it oblique.
See my answer above: https://forum.duolingo.com/comment/28634668?comment_id=36185386
The location (in such examples) is the verb's direct object - this example is in the imperative mood (देखी) so the subject (तुम) is implicit; object is 'behind you', and the verb is 'look'.
It's grammatically accusative; English nouns don't decline so we just say 'you'; in Hindi the oblique case encompasses the accusative, hence अपने।
That would probably require a mirror, and mean inspecting the body part 'back', not the area behind you.
Perhaps the confusion is phrases like 'watch your back'? That's sort of slang/a colloquial saying (maybe military in origin? Six becoming 'back'?) that does indeed mean 'look behind you' (or be careful of what is behind you) - but that is not conveyed by 'look at', as with many sayings and slang in general, seemingly slight changes to words destroy it completely, because they're often set phrases only said in one way.