Perry's Paint Fable comes to mind...

[David Honig]

At 03:50 AM 12/14/00 -0800, R. A. Hettinga wrote:
>> FOR ALL TO SEE
>> It's a spray which renders sealed envelopes transparent, making the
>> letters inside as easy to read as postcards. "It leaves an odour for 10
>> to 15 minutes," says the spray's inventor, but, apart from that, "no
>> evidence at all" that it's been used. While the manufacturer describes
>> "See-Through" as a "non-conductive, non-toxic, environmentally safe
>> liquid", human rights activists believe "it's an ethically questionable
>> product" which could tempt security forces to bend laws.
>> http://www.newscientist.com/news/news.jsp?id=ns226930
>
>[Lighter fluid and similar stuff works just fine. That's been known
> for over a hundred years... --Perry]

Wouldn't this be detectable if you scrawled on the envelope with
an ink succeptible to paper chromatography in that solvent? 

You can make primitive (before cheap float glass) windows by oiling paper...