nope. for example i look for "Re: your details" and the message body does not contain this phrase but the subject does - the email passes through the scanner...
You are right, the scanner only scans the MIME blocks inside the mail (including the plan text) but it does not scan the header. Would be a nice feature if it were configurable (scan headers on/off), or maybe a third set of expressions for the header? What do you think?