I've heard of someone doing this too with a retriever dog. It does sound crazy especially with the dog tortoise horror stories, but in this situation many other things can go wrong too, i'd say go for it, follow the dog closely but she may very well be able to find the tort when you cant because "something wont be right" somewhere smell wise.There is one more idea... Hear me out... Before my tort I have a RES... I have a outside area for her. I never would have thought that she would climb a 3 foot fence but she did. I have a very curious dog that had to watch her all the time. Well make a long story short. The day. She disappeared I took my dog back there and kept asking if he knows where Betty is? Find Betty! I swear on my turtle.... He was all over the yard. When I was about to walk away he kept digging and barking at the bottom of the deck where there is a small hole. Sure as hell... She was under there. Just a thought. Also depends on if your dog was ever curious of the tort. I know I know sounds crazy. But I swear on it that my dog found my turtle lol. Can't hurt to try.
And especially if its still missing at this point after days I'd say its getting rather important to find him asap. Any food bits left out been munched?