A ton of possibilities with this, isn't there?
It could even have already been gouged in the door when it came to WHF if they bought it second hand. I don't think an answer will ever come. Although the use of capital letters is interesting.
I had assumed it was a built in cupboard of some sort rather than a free standing one, at least the only existing photo #101 seems to imply is.
Jeremy Bamber's parents were living at Whitehouse Farm by 1957 when Sheila was adopted, and the thought that a member of another family who lived there before them would have also hated living there stretches the imagination a bit. If June or Nevill had seen the scrawl on the cupboard, built-in or not, it would have been removed or painted over as soon as poss. IMHO
And if Sheila did it she may well have covered it up by a drawing or painting on paper taped to the door (as shown by the remnants of whitish marks framed around the words) to prevent either parent seeing it.
Or if it was an act of defiance she could have left it there deliberately, so that her mother would see it.
[When Jeremy Bamber was awaiting trial, Colin Caffell visited the farm with Ann Eaton. As he was sat on "Bambs" old bed in the twins' room (what was formerly Sheila's bedroom) Ann came in and asked if he'd seen the words scratched (not 'carved') into what looked like new paint on the wardrobe door. He recalled seeing them sometime before in the last year or so, and seemed to think that Sheila had written them out of frustration on an earlier visit]