Hey Lykrast,
I'm sorry to hear you are struggling to find the last piece. We have stopped working on this game and we unfortunately couldn't add a scanner to it before stopping. We're now working on it's sequel/full game called Muri: Wildwoods.
My best advice to find the last spot is to not use your aimed spray, use your regular spray as it can get dirt hiding behind rocks that might not even be visible.
I hope you can find the last speck and get some piece of mind ♡