Mr. Monk and the Red-Headed Stranger - S1-E12
Plot hole: When Monk explains to Miss. Mass how she had murdered Sonny, a flashback of the crime shows that she isn't wearing gloves when she leaves the note to get him to go to the alley. Had the cops checked the note for prints, they would have shown Miss. Mass' fingerprints on it and knew she was responsible for Sonny's death.
Suggested correction: Just because they lifted fingerprints off the note, there wouldn't be a way to identify them as belonging to Ms. Maas if her prints aren't in the system. As a witness (and ruled out as a suspect because they thought she was blind), they wouldn't have fingerprinted her just because the found unknown prints and wanted to rule her out.
What about gun residue on the hands?