If you're in a cemetery and see coins atop of a soldier's grave, don't touch it! Here's the background -
You leave a penny if you want to give thanks for their service
You leave a nickel if you did boot camp with the deceased
You leave a dime if you served with the deceased
You leave a quarter if you were with the deceased when he/she passed
This is a way to let the family members know that they are not forgotten and their graves are visited.