During World War I, British intelligence explored various methods of writing secret messages, including using invisible ink. Captain Sir Mansfield Cumming, the first director of MI6, reportedly discovered that semen could be used as a form of invisible ink that was not easily detected by traditional methods of espionage at the time, such as iodine vapor. However, this method was eventually abandoned because of obvious hygiene concerns and the unpleasant odor as it aged. https://en.m.wikipedia.org/wiki/Mansfield_Smith-Cumming#:~:text=When%20the%20SSB%20discovered%20that,masturbatory%20habits%20of%20the%20agents.