Chick's probably around watching you. bobbi - j & centrarchid gave good advice, if it's around will probably return at dusk, if not check in the brush around the coop area & trees. Good luck.
A squirt bottle with a strong, far reaching stream works good to set your 3 month old straight about not picking on the little one. Introduce them slowly, put the small in an enclosure in the coop with the 3 month old so they can see and get used to each other.