Gwen hears Ianto's voice call "Here Captain" and she pauses outside the door. Gwen holds a folder to her chest and wonders why Ianto would call Jack to him like that. "Now sit."
Gwen remembers that time she had walked in on them in the greenhouse and blushes. She would have thought that they would wait until working hours to do... whatever it is their doing. She turns to leave and walks into Jack.
"What are you doing here?" Jack asks. It takes a few moments to realize that Ianto was talking to an actual dog and referring to Jack as if he is one.
"Err, on second thought it can wait," Gwen says. She rushes past him. Jack watches her in confusion. Meanwhile Ianto rubs the belly of a large dog.
Of course it was a dog, Gwen thought. Get your mind out of the gutter Gwen.
I don't know what type of dog Captain would be. I think he would be a big dog but that doesn't narrow it down much.