Transformers: Fall of Cybertron
In Chapter V: Cut and Run, there is a certain breakable wall that the player can break via Jazz's Grapple ability which leads to a hallway with a switch at the end. If the player attempts to activate said switch it will transform into a robot and proceed to dance. Afterwards, a door behind it will explode revealing a hidden room.
Rumble and Laserbeak can be found as hidden cameos in chapter two and if they are shot at they will retreat. Additionally Soundwave can be found the same way but disguised as a cybertronian light fixture and he will transform, taunt Optimus, and then retreat if the player focuses their aim on him.