The IRS goes after cases they think will result in an adjustment. When they are acting on tips, the more info presented by the tipster, the more likely they are to open a case. BTW, depending on the value of the tipster's info, s/he may be entitled to a reward when the IRS collects $$.
This post does not constitute legal advice, nor does it create an attorney-client relationship. Postings are based only on the information provided and you should consult an attorney in your area before relying on information contained in this post.