Finally found the cluster!
I peeked under the cover of the hive this afternoon, and found the cluster quite easily: smack in the middle of the upper super. My understanding is that this means that they've pretty much eaten everything else. I'm not sure how fast they'll go through the 8 frames that are there (all they filled before Winter), but I'm starting to think about feeding in the near future. My mentor was right, and I'm certainly glad I added that extra super in the late Summer! I'll try to open the hive this week to get a better look at the situation.
I was a little dismayed to see a couple of drops of water on the screen over the bee escape. I guess I need to do a little better with ventilation.
Don't provoke a hive full of angry bees.