X200T Standby issue

[cutsoabaprap]

Hi guys, I'm new here.

I've recently acquired an X200 Tablet (7450). It's developed an odd fault where it doesn't go into standby when I close it, but it does wake up when I open in. Also it wakes from standby in tablet mode when I remove the pen.

I've changed and saved the power options for the lid in windows, changed them back but still won't enter sleep when I close it.

Can anyone tell me where is the sensor for open/close is located that triggers sleep/wake?

Thanks.