Was he maybe standing there, watching the entire process?
Yes.
And maybe that’s not actually Mutt’s house?“
I probably would too.
All this work for nothing: joke’s on him.
Maybe someone else has constructed a Mutt Dummy, and we’re going to get recursion.
I think DanV got it.
Yes, Mutt was watching the whole time. Jeff’s question mark signifies “Now what do I do? How do I explain my actions to Mutt? What does he think I was trying to do, and can I use that in my explanation?”
Comics I Don't Understand 
Was he maybe standing there, watching the entire process?
Yes.
And maybe that’s not actually Mutt’s house?“
I probably would too.
All this work for nothing: joke’s on him.
Maybe someone else has constructed a Mutt Dummy, and we’re going to get recursion.
I think DanV got it.
Yes, Mutt was watching the whole time. Jeff’s question mark signifies “Now what do I do? How do I explain my actions to Mutt? What does he think I was trying to do, and can I use that in my explanation?”