I'm dubious as to whether or not "rare" chances to spawn seeing rogues actually happened or not. More than likely, it was a rogue screwing up their invis and thinking they were hidden when they weren't.
If you feel like you have to implement it, something like 1% chance to spawn and self buff with see invis spell for mob classes that can cast that spell. I'm pretty sure that's why loathing lichs (100% chance) should see everyone, for example.
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