Unsure if this is already logged as a bug, but its quite obvious when playing on current Experimental branch.

When entering any vehicle which already has its engine on, the character will 'start' the engine again. On-screen prompt string is "Stop engine" however holding the action will instead result in the player turning the engine 'on' again. This occurs in all Driving assistance states and all vehicles.

Handbrake
The handbrake animation/action is also played when entering a vehicle (in Driving assistance 'None' this will only occur if the handbrake is toggled On/when entering for the first time and does not occur if the handbrake was already off)

I felt this is likely connected to the engine state issue presented above so I haven't submitted a separate bug report for this.