>> Perhaps you meant
>>
>>   \csname\ifnum 3>2\fi\endcsname
>>
>> which indeeds does \csname\relax\endcsname, an error.
>
> We should be fine here in expl3, I think, as internally we have
> something like
>
>   \csname\ifnum\numexpr3\relax>\numexpr2\relax\fi\endcsname
>
> in this scenario, and that is fine.

Precisely. If there were stray "implicit \relax", then they would
appear in x-expansion as well.

Bruno