I beleive because F# does not use octal for leading 0's (as C did) and without telling F# that your constant is in hex, it assumes decimal. This makes unicode constants work the same way as normal constants.

This is quite interesting. The language spec states:

Unicode characters in UTF-8-encoded files may be directly embedded in strings, as for identifiers (see above), as may trigraph-like specifications of Unicode characters in an identical manner to C#.

However, I don't see an equivalent of let trigraph = '\\' digit digit digit (lex.fsl) in the C# spec, nor does csc accept it, as your code shows.