I don't think anyone "snuck" eggs under her, more like she took eggs from someone

My (4BO) girls have a huge plastic dog house for their nest, sometimes 2 - 3 will be in there laying & I find them all in one indentation

I've had 5 Wyandottes lay in there at once but the eggs would be in one indentation.
I have read to separate the broody from the flock, then there are those that keep them in with the flock. If at all possible put a wire cage in the coop with broody & nest, that way she's part of the flock yet she has the extra help of protecting the chicks from the flock the first week.