British Inteligence Service during world war I considered using semen to produce invisible ink. That would not be detected by ordinary detection methods.
Secrecy is of course the most important principle of the intelligence gathering community. The problem however is that your enemy knows that your data is as useful to them as it is to you, perhaps more important actually. This is the main reason cryptographic technology was invented.
A Diary belonging to a senior official, Walter Kirke at GHQ in France during the First world war reveals that the chief of the Security Service, Mansfeild Cummings more affectionately known as “C” (not M, as James Bond films seem to want to refer to) had been making enquiries with London University to develop invisible ink.
This would be an excellent break through – you could write in plain text knowing that without knowing what kind of equipment or technique had to be used to read the letters your enemy could not interpreter the letter. The solution it seems was to start using bodily fluids in the form of semen to write the letter. The technique would allow one to disguise the fact that invisible ink was there as it is likely that writing on top using normal ink or writing offer the normal ink would not effect the readability of the letter. So anyone intercepting the letter would see say a letter from someone on the front lines to their parents about how much they miss them – it is unlikely a genuine sounding letter would be investigated further.
However there were some side effects one agent was informed he should only use fresh supplies of the ink after officers receiving the letter started to notice an odd smell coming with each letter. The guy who came up with the idea also became subject to jokes and had to be transferred.