This week we learned there was a hidden reference to Monica Lewinsky in an official portrait of Bill Clinton.
Six years after the portrait was unveiled, artist Nelson Shanks revealed the shadow near the mantelpiece Clinton is leaning on is actually the infamous dress worn by Lewinsky.
And in today's Independent, British artist Mark Roscoe admitted inserting a 'ghost tit' in a portrait of wildlife presenter Bill Oddie.
"Bill Oddie was a horrible person, quite awful actually, so I added a ghost of a bird with its name in Latin written alongside," said Roscoe.
"I was testing his knowledge as an ornithologist. It was my cheeky way of describing him – something along the lines of a 'long-tailed tit', Aegithalos caudatus – as he was 'long in the tooth' and it sounded like a bit of an insult."
There are also apparently further hidden jokes in Oddie's shirt, but you'll have to find them yourselves as the artist isn't saying anything else...
Inserting secret details into art is nothing new, however, as artists have been having private jokes at our expense for centuries.
The Ambassadors, by Hans Holbein the Younger
Take another look at that blur in the foreground from the lower left corner... (the effect is best observed in a gallery).
Yes, it's a massive skull.
The Music Lesson by Johannes Vermeer
This innocent-looking scene is actually stuffed with innuendo, from the instrument called the virginal the woman is playing, the way in which her reflected image is looking at the man, the aphrodisiac wine and the phallic musical instrument.
The Arnolfini Portrait by Jan van Eyck
In a tradition carried through to the 20th century by Jackson Pollock, Van Eyck scrawled his name on to the wall behind the couple.
Et In Arcadia Ego by Nicolas Poussin
This painting featured in the 2006 court case involving The Da Vinci Code author Dan Brown as it's thought to hold a range of secrets including the location of the marital home of Jesus and Mary Magdalene.
Isabella by John Everett Millais
This was only spotted as recently as 2012 by a Tate curator - a character in the foreground appears to have an erection. Maybe the Victorians weren't as prudish as we thought...
The Last Supper by Leonardo Da Vinci
OK, so Mary Magdalene is not in it, whatever Dan Brown might say, but other art historians have claimed there is a code above Jesus referring to a flood that will one day wipe out humanity.
Mona Lisa by Leonardo Da Vinci
500 years on, and we still don't know the identity of the woman with history's most alluring smile. But in 2010 the letters 'LV' were found hidden in her right eye, alongside some un-deciphered symbols in the left. We still don't know what they mean...
More: [Introducing the ghosts of the art gallery]3