Hidden text can occur for many reasons, most are in no way malicious. Text can be same color text and background, hidden in html commenting, stuffed in to alts and titles and hidden with numerous CSS styles. All of the common findings of hidden text, malicious or not are a violation of Google’s quality guidelines for hidden text.
Blocking my page in robots.txt will prevent indexing.. This is just not true, your robots.txt is a tool used to prevent crawling… not indexing. While blocking a page in robots.txt will prevent well behaved spiders from crawling these pages, it will not prevent indexing, remove a page from the index, nor block the flow of PageRank to the page.