Baby cold medicine? Why medicate if you dont know what you're dealing with. If it's environmental, simply eliminate the problem. Environmental could be pollen, feed dust, ammonia fumes from soiled bedding, cedar shavings etc....
There are many strains of different types of respiratory diseases...mild to wild. They could possibly have a mild strain. If you suspect it's a respiratory disease, you could contact your local extension office or state department of agriculture to find out if they can be tested in some manner to find out which disease you could be dealing with, perhaps bloodwork could be taken rather than a necropsy. Here's a link to respiratory diseases in poultry:
http://edis.ifas.ufl.edu/ps044
Take a look at Mycoplasma Gallisepticum (MG,) and Infectious Bronchitis (IB.) Since there is no foul odor, that basically eliminates Infectious Coryza.
Keep in mind it could be a mild strain, you might not see all the symptoms as mentioned in the link for that particular disease (MG) and/or (IB.)