New Year's Greetings!

I am using MS Excel 2007 and VISTA. I have a very large workship with many
row groupings. Strangely, I now see at the far left three numbers in boxes
instead of the expected two (the second box indicating Level 2), that is, not
only the intended Level 2 groupings but Level 3.

The problem is I didn't do anything to group at Level 3, except perhaps by
mistake. I have scrolled through the huge worksheet but I can't find any
indication that I mistakenly grouped a row to Level 3. So why the number 3?
Can any of the experts solve this mystery, or is it perhaps a bug
(considering many rows have been grouped)? Many Excel can't handle too many
groupings, whatever the limitation is. Thanks for your help.