WYR live in a world where monsters hide in random objects in your house and pop out to kill humans if they don't check the items first, OR live in a world where aliens are like fishermen? They hide in the sky and disguise their hooks as things in the world.

For the former, they hide themselves in things like bed sheets, or your jackets, toasters, game systems, etc. Undetectable. You could potentially kill them once they emerge, but it isn't easy. Its also completely random. They could hide in several items a day, or you may even go as long as a month without being attacked.
For the latter, they can disguise things as big as your car, or things as small as a door to a building. Even pieces of surfaces. Roads, sidewalks. As soon as you touch it, you are impaled (but not killed) and slowly pulled into the sky. Sometimes re released, but if not, eaten.


Source.