I think one of our chickens escaped

OK- so I'm not very actively involved in the backyard chicken community here. But nevertheless, I live in a large village/small town and one of our chickens, I think it has escaped. I went out for no more than a few minutes around the block by our house, and I just checked the garden, in the coop. Looking at the chickens, and there is only three there. We have four. My Mum's going to kill me! Help!

UPDATE: It was hidden.. it was laying an egg! Oops