It was wedge between the back of the RHS headlight and the filler neck of the windscreen washer bottle. It may even be the source of the mystery noise I've been getting over sharp changes in road surfaces.



As the writing on it says, the rear is 33.5mm diameter - so it would plug a hole that size.
It's possible it has been there since the vehicle was made. Or it may have been dropped there in the last week when Suzuki serviced it?