I've just looked this up leeneia, and it does indeed have a whiff of BS about it. In the Wikipedia account, it mentions the case of a bank-robber, a Mr McArthur Wheeler, who instead of the more conventional balaclava, covered his face in lemon juice. Its properties as an invisible ink were assumed by Mr Wheeler to render his face invisible too.
Now either this is a load of the proverbial, or the unfortunate chap was as mad as a box of frogs.

One of my Siamese cats appears to think that if his little head is out of sight under his blanket, I can't see him at all. He tries this on if he's been having a wicked gnaw of our roast meat cooling on the counter top. But then he's a cat....