I had added a second box, peeked in and couldn't see any comb on the top bars, so took the box off, looked through the bottom box and put it back together. A few days later I checked and decided to make sure they hadn't started in the top box and found I couldn't pull a frame out. They had built comb from the bottom up using the edge of the bottom bar! I must say, previously when I took the top box off, thinking there was no comb in it, I wasn't all that careful with tilting it and setting it down. Being 90 degrees, the comb had fallen over. It was almost to the top, but not quite. I had read they could build up but thought that was just a little bit, not a whole frame. At first I thought that they couldn't get to the top bars to draw down. But this is a top entrance hive so they walk right past it all the time. Maybe different bees draw the comb. I guess this is where I need to move a frame from the bottom up to the top box.
I'm curious as to whether other foundationless people have experienced the same thing when putting an empty box on top.