Python, 88 
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actually reads the file
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I tried to do it the 'clean' way, turned out quite nice, although i'm still new to python and probably this can be shortened as well.

file named 'i.py' is the file executed.


    import string
    f=open('i.py').read()
    for c in string.printable:
    	if c not in f:
    		print c

And it prints out all the ASCII characters not found in source file. I know this was supposed to be a clever hack (as in all those which contain all the ascii set or addresses), but I wanted to do explicitly what was asked :]