Above is good answer. If you are unable to find a chick. Then integrate with the adults ASAP.
My story:
Now I have a baby raising tote to raise them in the coop with a "mama hen heating pad" (found info on byc)
The babies pictured are sitting on "mama".
Then: had a fox family make off with most of my birds just before a late blizzard (snow on the ground for 4 days in late may). Left with lead hen, 2 pullets and a cockerel, all but lead almost a year old. Ordered chick assortment for the next week.. fast forward. 1 cockerel in the bunch (good), they're in coop learning to live with adults. 6 weeks old, pullet goes broody, none are laying. Got a half dozen eggs from a friend. 1 hatches, rest were duds. Older chicks 9 weeks older (too big to work with the lone baby. When baby 3 weeks old, dogs attacked. They got 6 of the older babies, the broody mama and the rooster (last 2 now 13 months and the pair in my profile pic, he's on the ground not the steps behind him, btw). I now have lead hen, another just over a year old, a 12 week old cockerel, 7 pullets of the same age and a 3 week old broody raised chick who isn't fully feathered out. I gave her a heat source and started taming her, but left her with the flock. She's now stepped past the older birds and taken flock queen when the previous passed end of the summer.
My point: babies actually integrate better with adults when they're tiny (the learn the flock calls easier) and the older birds are more willing to look out for the baby.
The babies can escape into the tote and still be seen.