What is this sinister thing in my house? Should I be worried?

[ruddyturnstone]

We bought an old house just before Christmas. When we moved in we discovered this on the wall, hidden behind the curtain and pointing down towards the sofa. What on earth is it? Our best guess is it's just part of an old defunct security system but it's hard not to be creeped out by it.

We've put the curtain back over it for now.

It looks like a PIR detector for a security alarm.

A porn film maker is spying on you...

It's the same as the motion detectors in a house we lived in years ago.

Thank you, thought as much. I've never seen one that looks like this before. I still feel like I'm in 1984 though.