[thejeff]

No, I've never seen it.

Going through all the criteria just to be sure.
Some should be blatantly obvious, but I'm trying to cover all the bases.

Let's see: You own the province.
You haven't searched it manually, even 1 level in that path.
No sites found in any path? (less than 2 really, including mines and the like)
Lowest numbered province meeting those criteria.
Gems left at the end of the turn. (searching takes place before gem income)

What do they target when you set them back to searching? Their current province?

If your capital has only one site and is the lowest unsearched province, then yes your mages will try to cast on it every time, until you let them. But if they're going back to defend it shouldn't be that.

(I've been thinking of a mod that just adds empty sites to all the one site capitals, just to avoid this.)