A relative's dog, who only know's me a little, will follow me on a walk but on the way out will stay behind and then as soon as I turn back home will go ahead. Is this simple submissive/safety behaviour, keeping escape home clear?
Possible answer 1) Yes. Kind of what you suggested. He doesn't trust you as a partner, so while he respects your strength (following you), he keeps a escape route because a strong leader could also be a violent leader.
Possible answer 2) Also, it could be easier. It's probably that the bond with the original owner is so strong that the dog just don't like to modify it's pack. That would explain the "start at back, return at lead".
Dog's position inside a pack, during a walk, will always give you a lot of information about how it feels about you.
This, if problematic, could be solved in a variety of ways depending on dog's personality. You could be more "lovely" if the dog is coward. Or you could be more active during the walk, because effort will make dog to change its mind and focus only in you.
Its purely speculative but many dogs tend to favour a fixed route, with some minor variations. If the early part of the route's unfamiliar - they might let you lead (and classic dog training is that the owner/walker leads, not the dog) - and they might find that when they're closer to home or on the way back, they're familiar enough with the route to lead, not follow.
Depends on the dog and the walkier - but I do think its a function of how confident the dog feels, with a more confident dog leading rather than following. On the way out, they're seeing where you want to go. On the way back, they know where you're heading back.
I don't think it's anything mysterious. The dog probably likes you and wants to spend time with you or is curious about where you are going. If your relative doesn't take him out much he might just like the company - terriers, although small, have a lot of stamina and like spending time running or walking out doors.
When you go for your walk, he doesn't know where you are going and so has to follow. When you are on the way home, he knows the way, and so he can go ahead.
It could be as simple as that.
If he tends to keep his distance and you would like him to stick closer to you, try carrying treats and rewarding him for coming close.