What are those little plastic disk things hanging by doorways?

Question

It's been bugging me for AGES.

What are they?

They are blue plastic disk things (about 4 or 5 inches in diameter) and they have a smaller round sticker in the middle, looking like a round blue eye. They are usually by front doors and my neighbor has one...

Here's a picture link :

http://i35.tinypic.com/34zi0kn.jpg

Answer 1

i don't know. but it's creepy!