Search, Actually

It was a search check, not a spot check.

Kaspar finds the head of a crossbow bolt. Apparently the killer retrieved his bolts, but the head of one broke off after passing through the boyd.

The head of the bolt has a craftsman's symbol on it - or half of one to be precise.

You don't have Knowledge Local or Knowledge Heraldry, but Constable Einian or Scowlen D'Anvil probably do.