I have seen them spawn at night during fog after beating 3rd boss I think. You know you could look at the wiki instead of having several people making assumptions, even more funny when they haven't actually finished the game. I'm sorry but that's not exactly helping. So after having a look at the wiki: Yep, nightly spawn. As for during the day: IDK, could only have been from an event then. But I still believe the area wasn't fully covered. Edit: Sometimes after an event they will also stick around outside the base for a bit, doing a bit of hunting for you.
Also I understand one might not want to accidentally spoil themselves when scrolling through the wiki. But might be better to not actually consult the forums other than asking for help as most people do not use spoiler tags - or the topic says it all already.