Wtf Is This???

OK, so I have been recommissioning my 2002 Speed Triple and the bike was running well, had to change a sensor out but nothing major.. So after a hot ride out yesterday I stopped the bike and liquid started hissing away at the top of the engine and leaking onto the floor.. first thought was head gasket, but it was too high up for that... anyway it stopped quickly and not a lot of it actually came out, it was blue coolant. So today I decided to top up the coolant but could not find the top up reservoir... should be under the seat.. nope.. so I followed the small hose from the radiator cap flange which should go back to the expansion tank... but it goes to this strange metal canister which looks like an emission filter or such?? And the other small hose from the canister was just sitting under the tank waiting to pour all over my engine, then freshly painted garage floor.. does anybody know what this is?? Its definitively not an expansion tank and my bike is missing one.. also where does the other hose go to that would be on the expansion tank?? Help appreciated as ever!!

 

Thanks, its definitely not a standard part, I have ordered a standard expansion tank and will swap it out!!