This does not sound good at all, leaking religiously is never a good thing
You need to fill the coolant to the top of that filler cap under the tank,
very slowly, so that you don't get air bubbles in the system,
sooner rather than later!Then with the cap still off start the bike and
gently rev the engine to get rid of any trapped bubbles for about 1 minute, then top up again if needed.
As for the rear expansion tank, it should be no higher than the line in the middle when the bike is upright.
Short trips don't have anything to do with it, once the system is airtight the fluid won't really go anywhere and your levels will more or less stay the same all the time. You should check fluid levels regularly anyway.
If you seriously can't find any leakage anywhere, my next suggestion would be a radiator flush, because if you have a scaled up system that can cause the bike to overheat, then boil and dump the fluid.
Fill it back up first though then examine it again after you've ridden it, it may not be leaking because there's not enough coolant left to leak!
If you haven't got a Haynes manual yet i would seriously suggest you get one. You can download one on here to browse through but it doesn't beat having it next to the bike when working on it.
Nobody likes a greasy computer!