4 year old male neutered indoor cat got outside?

Our cat has been an indoor cat for 2 years now and we were looking after him at my dads house while mum was away. I am often round my mums so he knows my voice and smell but he has never been to my house before. He was due to go home today however this morning he somehow managed to get out. He has never been out before and we can't find him anywhere! Its been about 7 hours since he got out (we didnt see him escape) and he has not come back . We have called the local police and posted a reward and fliers. Has anyone had this happen to their cat and have they returned safely? We are worried sick!!