I've had indicators in the past of my luggage being looked through without chalk and without the white TSA card. I do still see the TSA card from time to time, but probably every other flight my "indicators" are out of place without the card.
My "indicators" are the alignment of the front zippers. I put them all in the center. So the main zipper, top outside pouch zipper, and bottom outside (larger) pouch zipper are all in alignment. I pick up my luggage and many times I see that 2-3 of the sets of zipper ends are no longer even close to being lined up, but all the way to one side. Clearly opened and re-closed, but no TSA card.
I've heard that in some countries they will mark a suitcase with a chalk "X" to show that it was x-rayed, but they put pretty much all of the luggage through the "x-ray" machines in the US, so it is the rule here more than the exception like it is in other countries.
