I have personally seen one of my old hens <RIP Momma!!!> stealing eggs from another nest to add to hers. She was sooo sneaky!!! I had been trying to figure out HOW she was doing it, and what she was doing was this :
1) wait for other hen to lay and then leave
2) go into that nest and try to abscond with the egg.
3) She would roll it to the edge of the nest
4) Hunker down and set her wing OVER the egg and then hold it tight
5) Slowly work her way back to her nest with the egg held TIGHTLY under her wing
6) Get back to her nest <which thankfully was close by> and settle down and relax the wing so the egg would drop to the nest
7) Tuck egg under her with the others and look smug.
I was in shock when i actually saw her doing this. I kept trying to figure out just how the eggs kept appearing in her nest when she didn't LAY that shade of egg. We would find eggs that had been dropped during the "egg-moving" routine and were so perplexed on how or who was kicking the eggs out.
Took over 2 weeks but we figured it out finally lol She actually pulled it off in front of us 2x. After that, we just set her in broody jail and gave her a few golf balls....then chicks at the appropriate time. Never did it again after that and she passed away when the chicks were about 12 weeks old or so. She was an old sneaky wanna-be momma