As far as Rider's ability goes I intended it to be a kind of general "see through illusions" skill, with a bonus to seeing through illusions intended to disguise a person as something else due to the fact that the skill comes from the boon Athena gave Diomedes to see through the Gods disguising themselves as humans during the Trojan War.

It wouldn't help with presence concealment but if he was looking straight at someone I'd expect him to spot them. I suppose invisibility would count as an illusion to disguise oneself but the argument could be made that either because these are some kind of phantasmal beast and not a person, or because he can't actually see the illusion to see through it, he's at a disadvantage in this case.

[@vancexentan] I'm fine with these creatures being able to ambush us, but as has been said there have been a few attempts by our Servants to scout the forest, such as Archer's vision and Caster's zombie rat, that haven't been acknowledged. If they don't find anything that's fine, so long as the efforts are acknowledged and we are told that they failed.