I have been asked this question, so here is the answer.
A knight in disguise is equivalent to an off-duty ‘police officer’ in a way. Although he is probably dressed up in normal clothing and his behavior may be a little more loose, a knight is still a knight. If he witnesses a crime , he may intervene, or he may not. Up to the knight really. Howver, unless you personally met him and recognize him, you will not know that he is a knight unless he identifies himself as such.