I have had chickens that have had what I'd call a wet sneeze and maybe a tiny bit of watery eye. Usually, it only occurs in the wet spring time. I am sure to open all windows and doors (if feasible) in the coop. Make sure the coop is clean. Ammonia can build up, especially in the wet spring time. It can cause respiratory issues. My first spring of having chickens I had this issue. Since then I am sure to open up the coop and I've never had any trouble since.
There are diseases that afflict chickens, too. If there is a smell to the discharge that is not good. If that's the case, it's called Coryza and it very contagious. I am hoping someone with more chicken disease knowledge can hop in here for you.
Good luck!