Many people switch off when you start talking about pheromones and other biological problems that are often associated with rats and other pest animals, but ignoring pheromones — or what leaves them as a method of communication for other rats — is going to be a bad move on your part.
1 — Pheromones are what rats use to communicate with each other. All the while Pheromones are still present in a building, other rats (and potentially other animals) can pick up on them, learn from them, and potentially even start up a brand new infestation because of them. Rats will pick up on Pheromones that have been left not only by themselves, but also by other rats. They will use the Pheromones to return to nesting spots that they once had abandoned, too.
2 — Pheromones are released by rats in a couple of ways, but the most common way is via urine. Rats pee and poop as they run around, which means that both urine and feces can be found in almost every single place that rats have been. They are not like raccoons, who tend to centralize their waste material in a single latrine area.
Essentially, wherever the rat has run around, it might have peed, or left a droplet or urine that it may have touched with a paw. Wherever the rat has run around, it will have left Pheromones. Those Pheromones are going to be picked up on by other rats, even rats that were, until now, presently outside of the building.
3 — Leaving waste material — urine, feces, bedding material, etc. — is guaranteed to invite in not just other rats, but other wild animals too. Rats are food for a large number of bugger predators, and they are likely to be lured in by the thought of a tasty meal. This means that even when a rat infestation has been evicted, the scent and pheromones left by the urine could still attract snakes and other predators. Those predators will believe the rodents are still in the building, hence why they attack — they are looking for food, just as the rat was before it.
In reality, not clearing up properly after a rat infestation might not seem like much of a big deal. The truth is, however, that not clearing up properly after a rat infestation could end up taking you right back to square one again — with a big infestation that you can’t get rid of.