It will be most likely your exhaust fan, if there is no other place visible.

They love to live in the guttering and under the lip of the roof. Inside hollow chair legs even, they don't need a lot of space. Get someone out to spray your house outside and inside the roofspace. Costs about $200 or so, but once they are in the house itself, you need to do it.
We had hundreds of them in Perth, all around the house and things, never really bothered that much until I found them IN the house! That wasn't cool. But the spraying lasts a few months and works really well.