For all you know, a surreptitious alien could be invisible. It could lurk, where it wished, as in closets, and under beds. If it wished, it could inhabit both creatures and spaces. Dust mites might be its eyes; spiders its ears. It may hug a wall, or rest in the weave of your pillow. When you look for it, you don’t see it; when you feel for it, it moves out of reach.