So 2^8 is a warning, but 2^BITS_IN_BYTE is not? I don't think whether or not the preprocessor helped in making the expression is a good heuristic for whether or not it is a mistake.
A warning heuristic needs to have a low false positive rate; a low false negative rate is nice but is not necessary. The purpose of a warning is to detect some common errors without inconveniencing too many correct programs. If some other errors go undetected then that is a shame but at least it is no worse than the current situation.