At one time Devonport Docks (Plymouth) were blanked out from 1:25000 scale OS maps. Not sure if this is still so. Looking at the map it was very obvious this was a secret place and no doubt a prime Russian target back in the Cold War. Blanking out the area drew attention to it - the reverse of the intention. Doh!
I make no judgments, just sharing an interesting YouTube link. I assume sharing this publicly available YouTube video does not contravene any state secrets. Mark filmed this for Channel 4 TV here in the UK.