We've lived in this house for nearly three years, but I've never figured out what this 'thing' is. I'm hoping someone can shed some light, please. :)
- It's on the wall, halfway up the staircase, about a metre up.
- It's white, rectangular and about 15cm high.
- There's a flap on it, with the word "BEAM".
- Behind the flap is a circular hole in the wall. There doesn't seem to be much else.
A friend said it was a central vacuum system. I don't know what this is, and I don't know how that would work! 
Any ideas?? Thank you!