I had weird stuff happen hanging out near the 1st box location as well. I opened the ?box (didn't take the weapon), ran straight outside (I was being chased by a slow one), turned right around expecting the zombie to be there. Instead, he was still walking to where I was standing near the mystery box, THEN turned and started coming outside.
It reminded me of Verruckt when sometimes the zombie looks like he chases you, you stop and fix a window, and the zombie keeps going.