Done, and now I understand that it was all about the browser used. Good to know. Thank you.
I have just tried this again with FF21, Safari 5.1.7, Chrome 27 and IE10 (all on Win 7 and same 23" IPS screen), the shadow is still much easier to see (against the 'dusty' surroundings) in any of the other three, except IE10, where, although it is (now) visible, it's nowhere near as obvious.
My screen is uncalibrated, but I trust it.