sick hen, don’t know what’s wrong

[27feathers]

On the other side, I’ll be monitoring the rest of my chickens more carefully. What early warning signs should I be watching out for?

 

[Eggcessive]

It is good to know if they are still laying eggs. Check the crop early in the morning to make sure that they are emptying overnight. Look for any swelling in the lower abdomen, runny droppings, or lice and mites. Make sure they are drinkin and eating well. If there is any lameness or unable to stand, see if one side is worse. Hopefully, this was an older hen with reproductive disorders which is very common.

 

[glassdragonfly]

So sorry for your loss

 

[ChickenCanoe]

With so few symptoms, it could have been any of 100 things and without a complete lab workup, there is just no way to know.
If you are to lose another, I would sent it to your state poultry lab for a necropsy. Especially those at a teaching veterinary college.
A veterinarian will cost more and may not be as comprehensive.
A veterinarian usually won't do all the cultures necessary to get at the root of the problem.
This is not to be critical of all vets, it is just that the university is better equipped to do the job, they should have an expert avian/poultry pathologist on staff, that's all they all day every day do and they do it cheaper.
Most vets will send the carcass off to the lab and charge a fee for the convenience - and that extra step will delay getting the results.

That was why I asked what state you are in.