I do not know enough about a convention where there is no clear winner, can a candidate who has not been mentioned or dropped out become the candidate?
Does anyone know how the process work?
Read the history of how James Garfield became president. He was the textbook example of a dark horse candidate. It took well over 30 ballots at the convention to finally nominate the guy who said he did not want to run. Sound familiar?
I would love to see a convention like that.
It may well be an interesting read, but it's not too terribly relevant now. The convention process is considerably different than it was in the 1800s.
To answer the original question, I know of no rule that prohibits the selection of a person who has dropped out or has chosen not to run. As a matter of practical reality (given that a substantial number of delegates are bound either permanently or through a number of votes), it won't happen.