The Footprints Mystery (Colour Jets) by Andrew Donkin - old paperback
Age : 7-11
LET YOUR IMAGINATION TAKE OFF WITH COLOUR JETS Irresistible stories for children who are beginning to enjoy reading alone. When strange footprints start appearing all over town, Tom is determined to find out who or what is making them. Unfortunately, so is Robert Smugg -- a professional tracker who lives up to his name. Who will be the first to solve the mystery?