Are the girls preparing to draw comb on the outside of hive?

I agree with @Dee, they are washboarding. They are not preparing to draw comb outside the hive, don’t worry. You just have a load of juvenile bees who are bored and just looking for something to do, so they dance - like human teenagers :blush: Here is a nice little video discussing it:

You may have a defective SBB - I did. If the wires are slightly bent, the bees can get through it. I had to bend back and reposition about 20 or 30 wires. Here is the post:

Until you are able to take the SBB out and check it, I would leave the corflute slider in the upper slot, otherwise the bees may decide to use the rear of the hive as another entrance, and going through the screen will deprive them of a lot of pollen too.