A Platform Sensor is a logic component that is Developer only/not available to the general public. It is was intended for exclusive use in the default Story to improve frame rates on the PS3 without making two separate versions of the level for both platforms. Players have gained access to it and other tools by editing the adventure mode levels using a secret menu which was made inaccessible in version 1.05.
Like the other objects intended for use by Sumo Digital only, when you attempt to highlight it for the first time in a session, LittleBigPlanet 3 kicks you back to your pod. It is unknown why it does this, however it was never intended to be used by players.