I've seen some VTL through yoga pants, since a lot of women will wear yoga pants or leggings for comfort. But like makke said, a lot of comfortable clothing and comfortable underwear, so unless they're really devoted to no panty lines, a lot of girls will probably wear normal panties. Another problem is a lot of them have big backpacks on, and the chairs at the terminals usually have full backs so you can't see anyone's back.
That being said, when I was at the Gatwick airport in Britain, I saw at least 4 see through leggings, one with a bright red thong underneath. Other than that, I've never had much luck either. I also saw a bodysuit whaletail at the Las Vegas airport but otherwise not much luck here either.