His parents were going away for the weekend in a few hours. Why would they smoke pot in his room when his parents were only hours from leaving? (Remember this was before the guy delivered the keg too early).
Wouldn't you say something like "Not smoking now, man. My parents are leaving in a few hours. We can wait. I'm not taking the risk."
They had incense going which I think was supposed to be a screen, along with a window open.
Incense more or less would mask pot over the long haul, but not mid-joint or something. Even cigarettes would help, I knew a guy in college who would light a cigarette in his room when he smoked pot. In the minute or two around a toke or two, you could smell it but it was pretty masked by cigarette smoke.
The trouble with incense anymore is that it looks like you're hiding pot.