Which code points are escaped inside quoted atoms and strings?

Discussion
1

What is the policy to escaping code points inside atoms and strings?
Since this is a Unicode question, the ISO core standard might
be not extremly helpful. So what is the best practice?

I got in SWI-Prolog:

?- X = 'abc\xFEFF\def'. 
X = 'abcdef'.

?- X = "abc\xFEFF\def". 
X = "abcdef".

The 0xFEFF code point doesn’t get escaped. If I
take the idea that a quoted atom is also a printable atom,
then this might need some improvement. I was rather expecting:

?- X = 'abc\xFEFF\def'.
X = 'abc\xFEFF\def'

?- X = "abc\xFEFF\def".
X = "abc\uFEFFdef"

That a string is printed with \u … instead of \x … \
is to help JSON and JavaScript output. How can I change
SWI-Prolog behaviour? Are there some flags?