Blue Peter 
noun
A blue flag with a white square in the centre, raised by a ship about to leave port.Manoeuvring my newly-articulated bike as gingerly as a first-time driver, I moved off with the flag on the back of the trailer fluttering like a Blue Peter....
- Whilst we were still on shore in Conquerno the Colonel, who was still on board, raised the ‘Blue Peter’ to the mast head.
- I was just about to raise the Blue Peter then and go without you, mate!