I travel quite a bit, but I haven't figured this out yet.

Every couple of trips, I will pick up my suitcase from the baggage claim when flying, and find that it has s decent amount of yellow chalk smeared on it. Not completely covering a single side, but still more than just a tiny bit.

These are all domestic flights within the US, and I mainly fly Delta. My luggage has just "normal" stuff in it (clothes and toiletries)...

Does anyone know what it is from, or why it is there?

It could be a mark for extra checking, but in previous years, if the TSA actually opens your baggage, they leave a little note inside explaining why your shampoo is now open and stuffed into your good shirts.

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.

My kid likes to hang out at airports and watch the baggage carousel. I let him, because its hours of easy entertainment. He's started taking his sidewalk chalk for those in-between times when the carousel shuts down. I'm guessing he ran along the carousel edge giving each one a yellow stripe.