Fozzy, As much fun as it sounds like you are having relocating them, maybe someone who cares is putting them in for you!!
More seriously, some references suggest that snappers are highly mobile animals and can migrate miles from one pond to another; a web search suggests that females will sometimes go a mile or more just to lay her eggs. I doubt one could outrun your truck back to your pond, but walking two miles back to a pond is not hard to imagine. On the other hand,
here is a link to an excellent discussion of snapping turtles which suggests that they only leave their home water to nest.
You might apply a little scientific method by leaving the next one in for awhile and seeing if two turn up. My own feeling is that they are no significant threat and add some interest to the waterscape, but they do unsettle some folks.
