I built myself three ventilated inner covers (This style)
and have been using them on three of my hives.
Haven't really noticed anything out of the ordinary with any of them, until recently. On one of the hives that was a single 8 frame medium using this cover, I added a second box of foundation on the 11th, 5 days ago. I want to say later that same evening that hive was very active, it seemed like a little more than an orientation flight, yet a little shy of swarming activity. But it was enough to get my attention, when I went closer for a look, the inner cover was FULL of bees, in a bit of a frenzy.
Anyway, they settled down eventually, but since then on several occasions I've observed through the vent holes that the inner cover has 100, maybe 200 bees all over it with 3 or 4 bees on each vent hole, like they're trying to get out or something, again, in a bit of a frenzy. Similar to what I saw that first evening, but on a much smaller scale.
Until recently, I've never seen this type of behavior with any of the hives with the ventilated inner covers. Typically when I look in the remaining two, I either see no bees, or maybe a small group of bees relaxed and chilling right at the whole in the center of the inner cover.
Also, the small entrance on the under side of the inner cover shows no activity while this is happening and the bottom entrance seems like normal light traffic with bees coming and going....
Edit: Correction, now that I think about it, I don't have the top entrance open on that hive....Maybe, I'll try opening it, not that I think that's part of the issue.