To which stream does xsel print the no newline warning?

xsel is a program with which you can access system clipboard from command line. If there is no newline at the end of copied content, it prints a warning after the clipboard content like this: $ xsel -b copied text \ No newline at end of selection Earlier I used to think that this warning is printed to the standard error, but today I found that the warning is not there even if the standard error is merged with the standard output.xsel-b |& less just prints the copied text, without the warning. Why does it behave like this?