"Dave Liquorice" gurgled happily,
sounding much like they were saying:

Mmm. Mebbe get some samples, and see what's visible and what's not.

If you just stick a patch on it'll be visible even if 99.9% transparent
in visible light. However if you stick something neutral to every bit of
glass in every window, it will be "invisible"(*) as all the window glass
will look the same.

Oh, sure - but it'll give an indication of HOW visible it's going to be,
compared to all the other windows of all the other rooms...

(*) Provided you don't go silly and stick a mirror film on. B-)

g