While I don't pick all 16 hooves every single day, they get checked frequently.
I do sometimes find the teeniest pieces of gravel in my horses hooves. They can be wedged in the whiteline or waaaay down in the frog, as in underneath the dirt I just dug out
I do my own trimming and imagine my surprise when I have thoroughly cleaned a hoof, and suddenly hear the rasp hit something that is not part of the hoof; something that I could not see with the naked eye to save my Soul.
As long as horses walk in pastures, paddocks, across gravel, down the road, "over the river and thru the woods to Grandma's house", there will always be miniscule pieces of stone picked up, some how, some way. The End