Recently Ive been noticing a bit of oil and dirt around the bottom of the motor on the ST1300. After investigating, found that the water pump has been leaking a small amount of oil through the tiny tell-tale drain hole situated just under the water pump...
In the photo below, you can see the tiny waste hole on the water pump, as well as the dusty oil around the motor... this oil hole is situated between the oil seal (engine side) and the mechanical seal (water pump side), so if either leaks, it will drain through this hole...
My ST1300 has done just 40,000kms, so not much work, but this is a fairly common fault, and if you see the bottom of your motor getting dirty, then this could also be your problem... very easy to get to once the fairings have been stripped off...