I appreciate that you may not know what has happened here but in order for us to help you, you will need to be more specific. When you say the 'right hand side' are you referring to the passenger's side (i.e. on you right as you stand in front of the car looking into the engine bay)?
If so, the coolant reservoir is located on that side of the car and if you overheat due to another issue, boiling coolant will flow into that reservoir from the radiator until the reservoir reaches capacity. Once that happens, the excess coolant. will flow out of the overflow pipe (located on the filler cap of the reservoir) and get deposited onto the road surface beneath (down by your alternator).