As Tahr has said, dogs can be very well trained and basically "bomb proof" but sooner or later there will be a motivation high enough that even a "100%" reliable dog will go and investigate/chase/root or generally bugger off from their "stay" point.
A few years ago a Neighbour from over the back came over to look for their 2 labradors. I asked did they get out the gate and the guy said gate wasn't closed as they never go out. Of course but then they did. I never heard if he got them back. Fencing, tying up or crating is the only way to be 100% sure ( well apart from slipping a collar, chewing the rope, breaking out of a flimsy crate) that the dog will stay there no matter how high the enticement to go for a wander.
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