I've seen Googlebot spider the contents of iframes.
Hello toolkit. When you say the above, was the content within the <iframe> part of the site? Or, was it content being pulled from off site? I've not seen Google index anything within the <iframe> itself but I have seen it grab content that resides between <iframe>Alternative Content</iframe> which is where the suggested alternative content goes.
If the page that populates the <iframe> is on site, is that page linked from anywhere else like a site map? That is the only way I can see Google indexing the content.
The content was part of the site, and to my knowledge was not linked to from anywhere else (it was the text for a scrolling marquee which used js to scroll the contents of an iframe). I suppose someone could have submitted the output page to google or have viewed the output page when using the toolbar.