I have a TOTAL mystery on my hands. Need ideas for what's going on here.

So I have a small flock of 6 backyard hens - all that are allowed by ordinance in my Tennessee city. No roosters are allowed either. After taking some time to figure out exactly what breeds I want , and after a very sad loss of three chickens to a predator (coop is now super predator proof), as well as some chicken-swappping with local permaculture friends, I now have two Silkie hens (both;aged about 9-14 months old, and one of whom is laying well.), and I also have one 12 month old Easter Egger who is also laying very well, even in this bitterly cold weathEr.
Approximately 2 or 3 weeks ago, I added two 16-20 week old Marans from the same breeder. One is a Blue Splash and one is a Blue Copper Marans. It seems unlikely that either of these teenage Marans have laid yet, and they both look very adolescent and gangly. They're definitely hens. They *were* both kept with Marans roosters before I brought them home. To add another data point, I have just built a small chicken barn, and two days ago I created two new nest boxes inside the barn using covered Rubbermaid boxes with holes cut in the front of each box. The new nestboxes sit on the floor. My hens have a secure covered run in addition to their little chicken barn, and they also get to freerange some around our securely (8 foot) fwnced yard when Jon or I are home to supervise. I collect the few eggs I am getting so far each day, often several times a day. Yesterday, however, I was gone from home from 8 am until 8 pm. When I got home, it was raining, and all the hens were inside their little chicken barn. I went out to close them in for the night and counted all 6 of them up on the roost except for the young Blue Copper hen, who had dug herself a nest in the straw in the corner of the barn. She seemed very anxious when I moved her to the roost. hens were up on the roost except for the new Blue Copper Marans pullet. She was huddled in the corner of the floor of the barn, outside the nest boxes. She looked very upset when I removed her from her nest on the floor and moved her to the roost. Well, that's when I heard a little "cheep cheep cheep" coming from somewhere inside the small, dark chicken barn. I was totally freaked out, so I called my husband out in the rain with a flashlight,, and sure enough, he also heard what sounded like baby chicks. We opened the top of one of the new covered Rubbermaid hen boxes on the floor of the little barn, and to our huge surprise, there were two tiny yellow newborn chicks, along with one brown/black one with a white dot on its head. They were totally alone in the nestbox and looked very cold. They still had dried egg yolk stuck to them, but no chipped eggs seems to be around. Jon and I initially just stood there in shock. Where did these chicks come from? We definitely DO NOT have a rooster. No way, no how do we have a rooster. Did these chicks come from the two newer very young Marans pullets? But if so, why haven't they been sitting on them in the 2-3 weeks since we brought the new hens home? How could these chicks have hatched without a mother sitting on them? Could a hen have moved them from somewhere else in the yard? Is that even possible? Since no hen seemed to be showing any interest in these poor babies, we wrapped them right up and set them up inside the house in a makeshift brooder. I've never raised chicks in a brooder before and have no idea what type of lighting to use. They seem to be doing okay this morning wiith a hanging 100 wt light, but do I need to get some other type of lighting for them? Can the lighting stay on 24/7? I'm feeding them medicated chick feed and water and they seem to be eating very well. WHERE THE HECK DID THESE BABY CHICKS COME FROM????? I am absolutely baffled. We've scoured the yard, and did find a few broken eggs here and there, but no other baby chicks.

This is truly a mystery, and I would really welcome any and all ideas you more experienced chicken folks have for where the chicks came from. Who are their mother and fatther? And how did they end up all alone in that cold, lonely, new nest box? Any theories here???