Just my thought process on the situation as I watched it.
Spoiler...
Like with the chandelier when Arkin first sees it - It was all bladed out but still on the floor. It was a trap the Collector hadn't finished setting up yet and the reason I came on that thought.
A definite loophole would be while it was being set up how the hell they didn't just happen upon each other in the first place, since the guy would have been going around the house to set up the traps. Oh, and also, since lots o people are mentioning the Collector's land record speed to set the traps up, he must have been pretty damned quite too - I mean, were the windows already boarded up - Maybe Arkin wasn't very observant, but he probably should have heard some of it... maybe.
Damn it - Now I'm starting to see some loopholes that I can't explain!
Spoiler...
He worked on that house for weeks. He knows they don't have an ill tempered, vicious dog. Wouldn't being attacked by that thing be a warning sign that something ain't right?!
Whatever - I still like it...