Martial Peak - Ch. 1335 - Opening the Door

[MangaDex]

 

[Kaarme]

Three possibilities, really:

1. It's not an ordinary arrow. Greenhouse flower Yang Kai just can't recognise it.
2. It's an ordinary arrow meant to divert any intruder's attention away from the real treasure.
3. The owner of the cave was a huge troll, leaving nothing but junk behind such a deadly array.