“I've never seen it happen with an enemy that hasn’t already spotted you before you cast invis.
Is this what happened?”
It is what happened.
ZV pretty much summed it up.
“It's happened to me before. I'll get attacked by creatures that just spawned, haven't previously targeted me, and haven't even seen me, while invisible. Same thing happens with G-Sanc some times, live to cope with it I'm afraid is all there is to do.”
The creatures were the spiders in Seilwood and the Orcs in the Hlint, North Outskirts. I haven't even entered the screen yet. The creatures were not attacked, had ever seen me visible or even in some cases, spawned yet.