"To notice any symptoms" means to become aware or recognize any signs or indications that something might be wrong or unusual with your body, mind, or overall health. Full definition
Sometimes, you don't even notice the symptoms of the problems your dog is experiencing before it's too late. (puppys-place.com)
While many women notice an increase in vaginal discharge during pregnancy, you're not likely to notice this symptom in the week or two just after conception. (babycenter.com)
Still, some people don't notice symptoms at all. (health.com)