As Moo would say Gato

So as most of you probably know I have to very hyper active cats. They're both rescue cats and have their issues that they have to work out. My oldest which is in the picture suffers from OCD and depression. The little one suffers from eternal hunger. She's always going through my whole kitchen looking for food. I've found my kitchen looking like Katrina hit it because she'll try to get into anything that might have food.
Last week my brother asked me if she could take the little one to his house to do some hunting at his house. He thought that they might have a mouse and wanted her to search it out. So she stayed with them for about a week. The house was a bit more quiet. Feeding time was less of a hassle. Life in my apartment was just a bit easier to handle. My older cat just seemed to be at peace. He was sleeping all over the house with out ever being attacked. She came back two days ago and I assumed everything would be fine. Now my two cats that never took a nap without one another were hissing at one another.
It's so strange to see how a few days away from home things have changed.
It made me think how as a person I can change when I'm away from my day to day things.  Once we're away from the known do we behave differently?