Highly likely that the cooling system could do with a clean. Once the waterways get a bit clogged up over time they get narrower causing the engine to get warmer than normal and with narrower channels for the fluid to travel through causing the bike to overheat.
When it overheats the fan will be the first thing to trigger, but after that if coolant can't move about properly it will start to rise in the expansion tank, and will eventually boil to the point it gets chucked out onto the floor through the overflow pipe.
When the bike cools down it would normally draw the coolant back into the engine, and a good airtight system, even on hot days, will retain similar levels of coolant every time, which can be checked with the bike on the centre stand then looking at the level to see if it's between the lower/higher markings on the expansion tank, looking underneath the seat from the right hand side of the bike.
Obviously if any fluid has been dumped then the chances are the expansion tank will look either very low, or just empty.
If this is the case then get a bottle of Radflush rather sharpishly and do a system clean out, then replace with new decent coolant.
Be warned, it is not an ideal thing to do in very hot weather, because you mix the small bottle of Radflush with pure water whilst going through the cleaning cycle, and as there won't be any coolant involved then the water will quickly boil like water in a kettle so bear that in mind.
You can either let the bike tick over for a while, or if it was a sensible temperature i personally would go for a 10 mile ride, but either way, in this climate expect the fan to come on again until the engine has been properly cleaned out.
Once done, flush out the shite out with a hose in the top of the rad and the lower engine bolt removed, then replace with the appropriate coolant mixture, or preferably (in my opinion) premixed coolant.
When you've topped up the radiator, plus the expansion tank is correctly between levels, with the tank supported firmly you can start the engine with the radiator cap off and let the coolant move around a bit to expel any air bubbles which will rise to the top. You can also VERY GENTLY apply a small amount of throttle to increase the circulation to help this along, then after a minute or so, switch off, let it settle, then top up again with coolant.