In Dark Souls, why do I randomly obtain estus flasks?

When I'm randomly traveling around Lordran, in situations that are otherwise unremarkable, an estus flask is all of a sudden added. It makes a weird noise and looks kind of like the animation for capturing souls from dead enemies, but it's gold. The only thing I've noticed is that it only happens when I'm not in combat, but I otherwise have no idea why it happens. I feel like this is a dumb question and there's an obvious answer, but I'd like to know.