Wouldn't call it trust, it's not as though Thurinar would give his back to these guys.
With his passive 20 insight, and 22 perception, Thurinar has identified the fellow in all black, who has already assaulted someone, as a shadow thief. It's an assumption, but one he is willing to track down and verify.
He isn't streetwise, so he doesn't understand the system yet. What he does know, is that the druid looks beat up, the bard looked beat up, and the trail of abused people leads to this door.
He expects the Sailor to continue the chase just as he would, that's another assumption. If the door isn't kicked in by his turn next round, he'll do it, and begin the chase again.