It's more like they are invisible scent emitting agents, you can tell exactly where the agents are by where pregnant Creatures will go in order to lay their eggs. For example there are two "homes" in the Norn Terrarium; in front of the learning room and at the top of the tree house. They also tend to go there when in need of comfort, not when lonely, and will say they are "homesick" when comfort drive is high.
ooh, never noticed the second norn home though now you mentioned it I know exactly where you mean.
Aside from laying eggs, and ettins' hoarding instinct, do they get homesick just by being away from home smell? I've never noticed. And do grendels seek out home too when laying eggs? I've never bred grendels much in the Ark.