HOT WHEELS UNLEASHED

Secret events are found in the Hot Wheels City Rumble mode and there are 9 of them to uncover. Secret events are marked on the map with a question mark and are easy to spot with their sparkling purple ring. It will also clearly state that they're secret events when you hover over one. Finding the events is the easy part, trying to figure out how to meet the requirements to unlock them is the tricky part, because some of them only give you a cryptic clue to go on, but fear not as I have the solutions below:

Secret #1 - Hidden To Most
Clue: Defeat the one that reigns the ice.
Solution: Win the Snow Pilot event.
Snow Pilot event location:


Secret #2 - Can You Keep A Secret
Clue: The legend says that a restless ghost blocks the access to this path.
Solution: Win the Ghost Seekers event.
Ghost Seekers event location:


Secret #3 - Secret Unveiled
Clue: Complete the Reckless Driving event using the Bone Shaker vehicle.
Solution: You will unlock the Bone Shaker vehicle after winning the Ghost Seeker event. Complete the Reckless Driving event using the Bone Shaker once you have done so.
Reckless Driving event location:


Secret #4 - Top Secret
Clue: Complete The Best event using the Buns of Steel vehicle.
Solution: The Buns of Steel vehicle can be unlocked early on in the Hot Wheels City Rumble mode. You will need to progress through a few events to reach it. From the first event in the mode (The Journey Begins), follow the road straight ahead until you reach a house with a green roof and a pink car parked outside. The reward box containing the Buns of Steel vehicle is located in this area. Complete The Best event using the Buns of Steel vehicle once you have obtained it.
The Best event location:


Secret #5 - Surprise!
Clue: Complete the Top Secret event.
Solution: The Top Secret event is another secret event that you will unlock once you have met the requirements for it mentioned above. Simply complete it once you have done so.
Top Secret event location:


Secret #6 - Reward
Clue: Complete the Amazing Drivers event using the Motosaurus vehicle.
Solution: The Motosaurus vehicle can be unlocked from a reward box on the east side of the map. From the first event in the mode (The Journey Begins), go all the way right until you reach an event called Tough Opponents and from here go straight up, passing another secret event along the way until you reach a reward box shortly after just to the left. Complete the Amazing Drivers event using the Motosaurus vehicle once you have obtained it.
Amazing Drivers event location:


Secret #7 - Covert
Clue: Reveal three secrets to take on the final challenge.
Solution: Simply meet the requirements of any 3 secrets to unlock this secret event. 
Note: It appears that the requirements for this secret have changed. The original requirement was to beat all 4 bosses, but that is no longer the case. I'm unsure why the change was made, but it's not a big deal and it's actually easier to unlock this secret now. 
No location necessary.

Secret #8 - Hidden From View
Clue: End the winged monster's reign of terror.
Solution: Win the Jurassic Predator event.
Jurassic Predator event location:


Secret #9 - Tell No One
Clue: Wake up, it's late! Today is your first day, you'll miss the schoolbus!
Solution: Complete The Journey Begins event using the Hot Wheels High vehicle. The Hot Wheels High vehicle can be unlocked from a reward box on the south-east side of the map. From the first event in the mode (The Journey Begins), go all the way right and then down. You will see the reward box in the bottom corner just past an event called Tough Opponents. 
The Journey Begins event location: