There's a strange patch of dirt in this greenhouse that's actually a grave. Dig around a little and you'll find the marker buried in there. If you dig deeper you'll probably find less pleasant things.
You decide to dig even deeper into the grave out of curiosity and you find a bow (ぼ) and some arrows. This must be the grave of a great archer! But you probably shouldn't keep digging around in this grave looking for more stuff.