Just when I think I've heard every possible stupid comment from a so-called "tech", I come across a thread like this one.
There should ALWAYS be coolant visible in the bottle, period. And as Don already mentioned, the volume of the cavities in the heads as well as the tubing remains constant, and the coolant expands when hot. Thus the level of the fluid in that bottle should INCREASE as it gets hot.
Check your fluid level cold, fill to the cold fill line. Ride the bike for an hour to get everything nice and hot, pull over and shut down, then look at the coolant level in the bottle. If it's lower than when you checked it cold, rather than the correct higher level, one of two things happened. Either there was an air pocket trapped in the system, or you have a leak. Leaks can be external or internal, btw. Some have had porosity problems in the heads that allowed coolant to leak into the combustion chambers, so if you see no fluid dripping on the floor, that's not a guarantee you have no leaks.