Python, 88 81 78 77 bytes

actually reads the file

I tried to do it the 'clean' (as in 'easy to read') way, turned out quite nice, although i'm still new to python and probably this can be shortened as well.

file named 'a' is the file executed.

from string import*
for c in printable:
    if c not in open('a').read():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, or use an error), but I wanted to do explicitly what was asked :]

Anyone has a suggestion how to make it even shorter while still going in this direction?

[edit] trimmed one more char, thanks for the tip!