Usually slugs eat a leaf that way. Maybe you missed seeing it in the dirt of the pot or maybe it hadn't come out yet when you looked or it may have been in the edge of the pot between the dirt and the soil. They hide under pots too. Lift up your pot and see if anything is under it. I doubt there's more than one or 2 or there would be a lot more damage. I'd spread a little safers slug bait around.

Slugs are a definete possibility i've seen them on my cosmos before, however just in case here is a picture for better analysis.

http://i23.photobucket.com/albums/b382/AFAuhhuh/Photo_062.jpg

arielletea,

Rather than being eaten, the tips of your older leaves appear to be dying. I notice the zinnia is in a pot of some kind. I suspect a nutrient deficiency. How are you supplying nutrients to your zinnia?

ZM

I agree with zenman. This isn't a critter concern, but a cultural problem: too much or too little of 'something'.