I have deleted several posts above (including mine) explaining the vanilla spotting mechanics and the (lack of) differences compared to the sniper-spotting mechanics.
I am not going to change the vanilla spotting mechanics, not even when asked by modders.
Chapter closed.
(Let's not go offtopic again and focus on sniper-spotting only.)
That leaves us with only one option... make the sniper-spotting mechanics different from the vanilla spotting mechanics.
I didn't want to do that either, but with modding interest, I shall reconsider.
I'll talk to Solarius about the details via some instant messenger and summarize here afterwards.
How about exposing getTurnsLeftSpottedForSnipers and setTurnsLeftSpottedForSnipers in Y-scripts the same way getTurnsSinceSpotted and setTurnsSinceSpotted are instead? I already have an idea of a sample script that could handle this "issue", but it's currently not possible without these operations.
We can do that regardless of this discussion.
I'll extract this into a separate task later.