melliebobs, we had a similar experience with our JRT x on holiday a couple of weeks ago.
Normally, on our boring old walks at home, her recall is very good. On holiday we were in Skye doing this beautiful walk over mountains with lochs below us. The path was wide and grassy and there were no livestock so I let the dogs off the lead.
Within seconds, Poppy had spotted something and suddenly just tore off. Running away from her was this enormous rabbit - double the size of any I've seen round here! This was just too much for Poppy. Nothing, but nothing, was going to make her come back. We called and called in dismay as we saw her run over the adjacent 6 or so mountains, fading into the distance.
DD1 and DH started running after her and DD2 was screaming and crying. We were getting nowhere and in the end I called them both back because I could see DD1 losing her footing and falling down a cliff into the loch. I was terrified Poppy would get stuck in a burrow somewhere or fall off the edge.
Then about 15 minutes later, back she comes, smiling like I've never seen before, pleased as anything with herself. We were all so glad to see her, but I put her on the lead straight away and she wasn't allowed off lead again for the duration of the holiday. There's no way I was going to risk losing her on a lonely mountain 700 miles away from home.
Now, back at home, she walks over our familiar fields as good as gold and her recall is fantastic. I guess the excitement was just too much! But the point is, our Sheltie would never do that. We can trust him 100%. But I think a terrier will do what a terrier wants to do, if the temptation is strong enough!