unknown death of hen

I'm sorry to hear that. Have you noticed any greenish yellow to white diarrhea? Has the hen been drinking more and eating less? In my experience Fowl Cholera can cause a seemingly healthy bird to die the very next day. Pasteurella Multicoda is the bacteria which causes it. Many people expect prolonged symptoms such as a throat rattle or swelling of the waddles and face. That does not always happen, and the bacteria is not uncommon, especially in warmer climates. Wild birds, and rats or mice getting into feed is a common way it is spread. Most disinfectants will kill the bacteria. It can be carried in yards on shoes, so a good biosecurity also lessens the chance of it entering the yard.

Preventing wild birds from eating out of chicken troughs, well drained land with no stagnant water also helps reduce exposure. Penicillin, Amoxicillin, Gentamicin, Enrofloxacin,Sulfadimethoxine, are all common medications used against Pasteurella bacteria. It is also a bacteria that often takes advantage of a bird with a weakened immune system. Birds that are cured can still become carriers, so prevention is preferred over treatment. I hope that helps. Perhaps there are University labs of animal sciences in Singapore you could have a bird examined if it happens again.