This happens a LOT on Harleys and there really is not any indication that you have picked up a stone unless you see it.
They do not slip and in most cases do not make any noise.
I'm guilty of not looking at my drive belt unless I'm servicing my bike, so a stone could stay in the belt for thousands of miles. I've not heard of a stone this size ever causing a broken belt. The good quality belts are VERY strong.
