I wouldn't call it cheesing to hostile someone and deny them the Perform bonus which might upgrade their Bardsong especially if combat is imminent.
That seems circular in that combat being imminent is the cause for hostiling, rather than anything to do with the Muse feat. It's the same as hostiling before casting a "Mass" buff to prevent buffing those you're getting ready to PvP.
I would call it cheesing, and would report them, if they detected a Muse via their Perform skill going up, even though the Muse can't be perceived. How can you be inspired by the presence of someone who is not, to your knowledge, present?
Report them for what? They can't help it if they're inspired if that's how the mechanic works.
I agree that it should work only if the muse is perceived, but it doesn't *have* to work that way if Devs don't code it that way.
How many players check their Perform skills as a way of seeing whether a Muse might be around? That seems like an almost never occurrence, and they would be doing it only if they already suspected it. So I'm not sure how the other player would ever know it to report it.
As far as the boost to crafting, yes, they're more likely to notice that passively, but they have to be at a crafting station and actually crafting. They're not setting out to detect the muse. So maybe they do notice that it seems strangely easier to craft.
I think the feat should work only if perceived. But if it works even when the muse isn't perceived, I think the instances in which players would be using it to detect stealth are negligible.