It will indeed have been caused by the oil seal under the injector pump "moving".
The seal itself is not actually available from Honda and is a strange size, but it is apparently a size used on a number of jet-ski/wave runner type motors, so take the pump along to a good bearing/seal specialist or water-sports dealer, and they should be able to match it up for you.
A thin smear of silicone/RTV on the outside of the seal before you press it back in should hold it in place securely.
As a word of caution, and easy to say with hindsight, I make sure the oil tank on my MC21 is ALWAYS topped to the brim - even topping it back up after a little jaunt to the beach and back. Therefore, if the level when I come to the bike is EVER lower than fully topped up, I would know it needed investigation before riding!
Andy.
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