THE WRITER'S MIND

I'm sitting in the car at a traffic light, and my mind wanders. My hero accesses the villain's fortress to determine how they are keeping open a dimensional rift. He's been there before but missed some important data. What is different this time? He has gone shopping in the interval. Did I have him buy electronics so he could use his spacefaring skills to assemble a sensor device? Oops, better add that into the story.  What data would he collect this time that he wasn't able to acquire...