Most likely, Sebastian has been out. Probably visiting Guinevere, or else figuring out how he's going to hunt down the particular target he's looking for right now.

It puts him in a slightly poor mood, so when he stalks into the hut his eyes are like chips of emerald (nearly literally) until he realizes that someone else is there.

"--good afternoon," he says, with a glance at Merlin that is a plain question mark.