Another point that has come up, that needs clarification: YES! an invisible person in a thick fog or mist or rain or even smoke cloud will be visible by his outline. No exceptions to this rule should ever be allowed, because it is simply physics.Remember that the Grimoire was published in 1977 and likely reflects rulings made by Hargrave and his circle before that date. Had it been published in, say, 1983 or thereabouts, I'd have considered the "it is simply physics" perspective to be very much in line with the kinds of articles you regularly saw in Dragon at the time. But, as you can see, it's of a much earlier origin -- a reminder that nearly all the schools/styles of gaming were present, to varying degrees, from the start of the hobby, even if many of them were outliers without much influence until later on.
I'll have more to say about Arduin in the coming weeks. As I've been re-reading it, I'm noticing that, moreso than almost any other published product from its era, it's primarily the record of a particular localized gaming "culture" and arrogates to itself no claim of authority beyond that. In short, it's a product by gamers for gamers "offered in the spirit of sharing," Hargrave puts it in his "forward" [sic] rather than a definitive statement by a game designer or game company. Much as I would never use at least half of what's in these books, I can't help but appreciate Hargrave's attitude.