What!? Your a a lier, no there is not XD
I will go look for them right away, though last fly around I took I did not find any hidden ways to get inside the other islands. The only other structures I cant get in are the ones on the cresent islands chain.

Edit: I have found 3 and been in 2 hidden rooms, the one on bteam island does not open with its open method (that I can tell) and so far I'm hunting for another.

Update: I finally timed the Bteam island correctly it seems, and I have seen the inside. I think that means I found them all?

@RomaDaddy the BTeam room is timed pretty hard or?

you have to get it just right, its a combo of the fact that when I land in the area i get double jump tossed into the wall and that sometimes my head would hit the dirt and keep me from getting in in time lol.

umm is this the current lobby i thought its the one with the white portals...