The more I think about it, the more I think this acts like a fireball without a reflex save. The simultationist in me hates the rules changing depending on who's in the area, but it's pretty clear. If you cast a fireball into an area without occupants, you remain invisible. If you cast a fireball into an area full of puppies, you become visible. Now, according to the rules, even if those puppies have immunity to fire damage the intent is to attack, and even if no damage is dealt, you become visible. Amusingly, if you know they're immune, you're not attacking, so you don't.
It becomes the ultimate metagaming rule. Does the player know/think the spell is likely to be harmful to its recipient? Does the spell directly impact the recipient? If yes to either, they become visible.
Summon spells don't count because the spell doesn't harm the target. The results of the spell do. Same thing for making a wall of iron and tipping it over.
So, sadly, IMHO lobbing this object at a bunch of drow makes you visible. Doing the same at a bunch of humans, not so much. Amusingly, lobbing it at a bunch of drow disguised as humans makes you visible.