Your discovery supports what Grimbriar's spy has been telling him. Grimbriar has arranged an exchange of important papers.
'I have a spy among the brigands who may be helpful in divining the true nature of this Sharkey. I will send him a message instructing him to come to a certain hill not far from here. Meet him there and ask him about Sharkey and his plans.
'The meeting place is by a grey rock in a small stand of trees directly north of Bree-town.
'If my spy has no news to share, we will need to find another means of learning Sharkey's plans.'
Grimbriar's spy will meet you by a grey rock in a stand of trees, north of Bree-town.
Grimbriar has sent you to meet a spy he insinuated into the ranks of the brigands in hopes that he will reveal information about Sharkey and his plans.
Watcher Grimbriar can be found at the stone house near the cross-roads outside of Bree.
The spy's notes further supported the brigand's letter, verifying that Sharkey is no brigand himself, but an influential man of the south. Grimbriar must know.