1999 indy classic 500,coolant leak

got a friends sled,that for the last few years he swears there is a coolant leak because the level in the overflow bottle lowers every time he tops it off and then rides for a couple hrs maybe 20 miles,sled has 5000 miles,only traceble leak is just slightly wet belly pan,but can not definitivly trace any leak,i told him possible head gskt or water pump gskt and or seal,any info on this kind of problem would be appreciated

1999 indy classic 500,coolant leak