It's most likely the area they feel safest in to be honest. Is there anything that could be particularly attractive to a hamster here? For example, a corner, ledge, underneath a table, etc. I wouldn't recommend letting all your hamsters out at once as this can be very stressful, and leaves the risk of illness spreading, and fights had they happened to escape. It could also be due to the fact that they smell one hamster who happens to be in that spot, and they decide to investigate.
Balls in general are not very good ways to provide you hamster with out of cage time in all honesty. Hamsters explore their environment by feeling things with their whiskers and sniffing. Being stuck in a ball unfortunately doesn't allow for either of these natural behaviors. Besides the aforementioned items, balls can get pretty hot and stuffy quickly, and dwarf hamsters are prone to getting claws and toes caught within the air slits. A much safer, and more stimulating alternative would be a playpen. These are very easy to make, and make playtime a much more leisurely affair! Check out this thread on a homemade playpen,
Stylish playpen
If you have any other questions, please ask
