A large helicopter formation will fly over central London later this afternoon.
IanVisits notes there's a short-notice air traffic alert warning of a 'large formation of mixed type helicopters' flying between Iver and London Bridge sometime between 2.40pm and 3.30pm on 19 April.
We know Barack Obama is due to leave the US today for a week-long international trip; we're willing to bet this is his route into central London after landing at Heathrow around lunchtime (he's been known to fly in these helicopter formations in Europe before).
Take a good look — you might spot one designated 'Marine One'.
Obama's due to have a belated birthday lunch with the Queen on Friday, and dinner with Will 'n' Kate and Harry at Kensington Palace on Friday night.
White House press secretary Josh Earnest says Obama is "looking forward to the opportunity to wish [the Queen] a happy birthday" when they dine together.