Teething. Teething babies often will put their hands in their mouths to make themselves feel better. Other signs of teething include if they are drooling as well, or tugging at their ears (nerves to the jawbone are shared with the ear which can lead babies to tug at their ears). It may be just exploring their environment too. Just make sure her hands are clean!
Pleasure. Lips have sensory nerve fibers that send pleasure signals to the brain. Newborns are prewired to suckle without ever having been hungry & the pleasure of touching the lips reinforces suckling even if nothing is there. Older kids & adults may still have enough pleasure from their lips to continue chewing nails, gum, smoking, etc. In short it doesn't mean hunger, it means they found pleasure.