Oh dear Den I'm hoping ur beloved terrier has just got on the scent of a female or an irresistable wild animals scent and will make his way home.Have you checked burrows or waterways with drains ?I know how much worry you must be feeling so am hoping that you have been able to let the neighbours know he is missing and maybe post fliers in the local town shops.By the way have you tried taking you're other dog out on the lead looking for the other dog ???just an idea.Prayers and thoughts to you and you're missing little bud please kep us informed on any updates.