Red-Blue had reached Building 2, and he had decided that perhaps this was where he would stay.  This may sound like an abstract, future-imagining sort of thinking for Red-Blue, but rats are often territorial, and when they are on the move there is nothing so near the top of their mind as "would this do for a new home territory?"

Building 2 was, you may recall, the sewage treatment building.  It had, for Red-Blue, several advantages:

1) ready access to food, albeit much of it not what a human would consider food any more
2) not too many people, to notice him and perhaps take hostile action
3) moderate temperatures, because the bacteria that actually processed the sewage needed to be kept happy and healthy
4) no cats or hostile robots

In point of fact, when Red-Blue had been in Building 3, which had a lot more food, the anti-rat programming of the robots had not been an insuperable problem.  They were not quick enough to catch a rat that was paying attention, so once Red-Blue had a few close calls, he understood what risks they posed and how to avoid them.  Their movements were sluggish compared to a rat, and they were not very suspicious; if he stayed out of sight they did not come looking for him.

The cat, on the other hand, was terrifying, and stealthy.

The first few hours in Building 3 had been thrilling.  Red-Blue had been followed by his friend Green-Red, and they had quickly been able to shrug off their earlier irritability.  They groomed each other a bit, and set off, whiskers a-twitter, to explore the Colony.  The first two buildings they had found access to were residential, and far too busy-sounding.  Building 3, at night, was empty (or so they thought), and the smell of abundant plants was enchanting (they were hungry by this point).

The first few hours, they had learned how to avoid the robots, and started to figure out how to nonetheless get to the plants on the tables, and then disaster struck.

Loki, the cat, appeared out of nowhere, and pounced on Green-Red, and his back claws raked open the rat with quick, strong kicks.  Red-Blue immediately began to run away, then paused for one moment as he realized Green-Red was caught, and almost turned back.  Then, with an anguished cry too high frequency for anyone human too hear (but well within Loki's hearing range), he turned away again and ran for the exit.  He darted from table leg to table leg, so that he had another direction to go at any moment if the cat was about to catch him, but Loki was occupied with killing, and devouring, Green-Red.

When he got to the tunnels below Building 3, and into a good hiding spot between the pipes and the wall of the tunnel, he stopped and listened, and shook with fear and grief.  Rats do not have much in the way of abstract thought; he did not think anything along the lines of "Green-Red, my best friend, is dead.  I will never forget him.  Curse that cat!"  He did, however, most definitely feel fear, and also grief.  Rats are often social, and Green-Red was the rat he had shared a cage with on Earth, before the two of them had been picked for the mission to Mars.

Then, as his emotions subsided, he continued on, and found Building 2.  It was, for him, paradise.  He had found it, he had found the Promised Land.  Food, moisture, moderate temperatures, no predators, not many humans.

If only he had someone to share it with.