AAhaa !91000 kms. did you own it since 0 meter or bought a used one??
May be a pin hole due to Gartry (Subframe structure being pressed slightly with the radiator lower belly wall) if car is hit from below while crossing a big pot hole. You won't find drop by drop leakage but upon deep inspection by workshop mechanic you will find some seepage in that particular area but all this can be done after removing below plastic shields.
This thing happened with me exactly like your car did and found a pin hole type very minor crack which my workshop mechanic diagnosed and rectified by applying imported steel sealant rather than welding and opening the complete radiator body. It was happened and done at around 60K and went fine till i sold the car at 94K.
As i used to check all fluids twice a month so once by looking coolant level low in reservoir i filled it with coolant again but got suspicious and there upon digging it out found what i mentioned above.
As you stated, the reservoir bottle should compensate that evaporated water and the bottle should have been empty. Look for radiator cap or some cracked piping and finally the coolant circulating pump called (water body). but all that can be verified by an experienced mechanic if you apparently found nothing.
Also verify Automatic fan function especially with AC both fans functionality.
