One problem with BTs, is they are known to have a 'change in their priorities' when out on the hills and can bugger off. A friend of mine lost his while out running and scoured the hills before and after work for a week but to no avail.
But everyday he always repeated the exact same route at exactly the same time as on the day the dog went missing, and after 7 days, the dog joined back in with the run as if nothing had happened. He hated to think what it had been up to.
On the plus side, BTs are easily small enough to lift over stiles and rocky steps. On really steep rocky or icy ground they will also happily travel in a closed up rucksac on your back as going down fox holes is their forte.