I’m guessing they think it’ll make a nice home on the other side - very clever those little blighters. Bit late in the year to be making a home so if you have them flying in & out at present some good & bad news………..
The bad news……..yep, they’ve established a colony within the cavity
The good news………….give it 6ish weeks and the colony will die out naturally, after that put some expanding foam back in the void and block up