If you have a cardboard box, pop that on the lawn with some hay and dried food in it ... piggies hate being exposed and hopefully he or she will find comfort in that.
Do you have a big garden? Are there any areas that it could wander to e.g. under fence, sheds etc?
Years ago my husband opened the hutch door and one of my baby piggies fell out. He didn't notice but hours later I went out into the back garden and straightaway noticed something moving under the hutch. Yes, it was the baby. He hadn't gone far ... frozen with fear! Checkout nearby areas to where he was last seen ... I am keeping my fingers crossed for you!