There is more than one method for hiding a layout object in FileMaker 14. What method are you using?
Things to try:
Use Hide Object When if you are not already doing so. Not only can you hide the tab control this way, but, if necessary, you can eliminate the tab control and just hide the individual objects within the tab control.
Change the layout theme. I've seen some "phantom lines" before that disappeared when a different theme was used.
Look for any "drop shadow" for the tab controls in question. If you are conditionally formatting the fill color and border colors, this won't affect a specified drop shadow.