Missing nostril flap?

[darj]

Hi! New here, unfortunately for an apparent injury. I have three young barred rock hens, all very healthy, generally get along well. This afternoon I went out and noticed my favorite one giving this odd, staccato sort of cluck every few seconds, breathing through her mouth off and on, holding her head up weird but walking around, alert, etc, with the other two. I held her and felt along her throat, which seemed fine, she doesn't seem to be in any pain. Then she turned her head and I noticed that the little curved piece of beak that should be over the top of her nostril is totally missing on one side. There's no blood and she doesn't seem distressed, and it doesn't look like the more dominant hen has been plucking out her feathers or anything that would indicate a pecking order issue. What's the deal? Should I be worried?