It was comforting to get a reply last night and we did put flour around the hutch - while we were doing this, we saw the guinea pig (although not near enough to catch it), so at least we knew it was still in the garage.
We looked this morning- saw some blobs where the guinea pig had disturbed the flour, but no idea which direction it went in.
Later this morning, we managed to corner it in the garage and have now caught it.
One other idea we had as a last resort was to to use a live rat trap that catches a rodent alive and conscious so it can be released elsewhere.This was as a last resort & we hadn't got one by the time we caught the guinea pig. They are both safe & sound & cuddled up together indoors now.