Each tab panel has its own fill and line settings. Could the line that you see be the border of a tab that you have not selected a line Appearance setting of "none"?
I have tried using only 1 tab I even went so far as to place the tab in a square object set the background to white for both and line to none and there is still a faint border
My preliminary tests made the tab border completely disappear. But it was on a classic themed layout. When I tried this on a layout with an FMP 12 theme, I got the same undesirable border you did.
I then clicked the "remove all styles" button (White with red slash near very top of appearence tab.) and then repeated the steps to set the line option to none. This worked to produce an invisible border.
Thank you very much it worked just fine did have to do it for each tab
Thanks again Jim