Wow Crazy.... on a plus you got more chickens but a huge inconvenience because your flock was healthy and now you have to deal with these issues. And Hopefully they don't have any other underlying conditions. I really feel for you and the rest of your flock. It does really sound like someone knows when you come and go. Maybe a little too much.
I have placed a lock on my run and Barn, I will tell you why....people round here been dumping animals left and right, and then someone tried to give me 19 chickens and I refused them because they were really ill and they were not really forthcoming about their condition until pressed about it, last month I found out 2 more farms in my immediate area that have chickens and rabbits have a bad outbreak of something in the respiratory and Cocci department. I was afraid that since we live on a farm and that most people in the community know that I provide care to my animals and I also have hens and rabbits that they would be tempted to drop them off because they couldn't care for them. And the last thing I need is sick animals making my healthy flock Ill with something. I also have motion detector lights at the entrances and the dogs pen is near by so they will bark if anything comes too close.
Just 3 days ago I came home and found a box on my doorstep, inside I found 2 guinea pigs and tons of blankets to keep them warm, It was nearly -20 out funny enough my neighbour was in his shed and saw someone pull up in my driveway and when I asked him he had seen anything he gave me the vehicle description and I knew exactly who this person was.
I ended up bring the guinea pigs back to him and told him next time he should try asking, and if he couldn't take care of them to put an add out on our pet rescue site in town. He was a bit peeved and I told him that because of the way he did it I was letting the rescue centre know that he just dumped them on my doorstep. He gave them up to the rescue centre the next day, But man, do people just not think about the consequences before they act??
I would have loved to keep the guinea pigs don't get me wrong, but I got males and these were intact females, beautiful as can be, but it was more about the lesson learned and the fact I have 37 animals.