If there are regularly badgers around, it could be an issue, but if there has just been one passing through, possibly not so much. You say the hogs all disappeared last year about this time last year, so it isn’t surprising it’s happened again. I have always found that the male hogs are first to disappear (all the males here have already disappeared), then the females and the hoglets are last to go.
But were the foster hogs rescued from your vicinity, because if not it’s better anyway that they are released back where they came from. There’s useful information about that in this link in case you haven’t already seen it: