What marks do professional editors and proofreaders use to indicate that a word is misspelled? There are many different sets of editing, proofreading, and typesetting marks and symbols, but the indication I am most used to, circling the word and writing SP in the margin, appears to be most often used to indicate that an abbreviation should be typed out fully.
A mark consisting of the letters sp enclosed in an oval is commonly used by teachers (and some professors) in the United States to call a misspelling to the student's attention. But as you note, the standard meaning of that mark under U.S. proofreading conventions is "spell out", and it appears in the margin of the same line where the proofreader or copy editor has circled a symbol, an abbreviation, or some other short-form word to be spelled out at full length.
Peggy Smith, Proofreading Manual & Reference Guide (1981) gives three examples of this "instruction to spell out"—in which the circled inline element is (1) the entire expression "3 in." (to be corrected to read "three inches"), (2) the ¢ symbol in "5¢" (to be corrected to read "5 cents"), and (3) the abbreviation "N.J." (to be corrected to read "New Jersey").
But proofreaders, copy editors, and typesetters don't have a widely recognized mark to indicate "misspelling"—and the reason they don't is that in a professional setting, markup is not done to point out mistakes in the work to its author for later correction by that person. Instead, the point is to correct the errors as quickly as possible.
So if a proofreader (using common U.S. style conventions) comes across the spelling millenium, rather than taking time to label it a typo, he or she will simply add a caret mark (^) beneath the line between the n and the second i and mark an n above the line for insertion there.
If the proofreader encounters the spelling senetence for sentence, he or she will draw a delete line through the extra e. For the typo recieve, he or she will draw a transposition line indicating that the order of the i and the second e should be reversed. And if the spelling is truly mangled, such as numoanya for pneumonia, he or she will draw a line through the entire word and write the correct spelling above it.
There isn't a mark that means "misspelling" by itself in "standard" proofreading marking. "sp" with a circle is short for to "spell out", which, like you said, means to expand an abbreviation.
Misspellings should be struck out with the deletion mark and the correct spelling should be written above it. Or if it's just a small error, like a single missing letter, it can be inserted with the insertion mark. A single unnecessary letter can be struck out with the deletion mark.
A list of marks with usages for reference: http://www.door24.org/images/textImages/editorsProofreadingMarks.png
But the "sp" with circle notation to mean "check your spelling" is really common in school settings. This is probably because teachers don't want to proofread their students' work (that is, give them the correct answer immediately) and instead just want them to be aware of the error and go look it up thenselves. As a commenter noted below, proofreading marks can be a matter of style or appropriateness for a certain situation.
I'm not sure there are many (any?) professional editors or proofreaders still working on paper copy. Every publisher I've worked with has expected a digital file from me, and the edits have been done digitally.
That said, the principle is the same - if a word is mis-spelled, the editor just changes it. That would be with "track changes" on, so I can see that the correction was made, but they don't mark it and expect me to make the changes myself. I just approve the change they made.