Old birds can get, carry, and spread Marek's without dying from or showing it's symptoms pretty often. My flock has had Marek's disease for over a year now and I have only lost two birds that were over 1 year old. I mainly just lose cockerels and pullets, their deaths usually would take a week or more but I just cull them once they start walking weird. I have had some birds get killed within a day or two but they usually show some extreme symptoms a few hours before death. They get very lethargic, can't walk, they feel all droopy, and their pupils burst into weird shapes. If two of your birds got Marek's then that coop is already infected with it and the old hen already has it two, it's on your clothes and in your house. Next time get vaccinated birds if it really is Marek's. You can have your birds tested for the disease. Until you get testing done I would not assume it was Marek's, there are plenty of diseases that can kill birds overnight and if you can't name any symptoms then we can't narrow it down to just one.
