"As far as we know we're the first in the world to train rats to be used in police investigations," said Mark Wiebes, who heads the Dutch police "innovation" centre.
Each rat costs just 10 euros (around $13) and can, in theory, be taught to identify any odour, from drugs to gunpowder, while a sniffer dog costs tens of thousands of euros to buy and train.
A police dog's training also takes around eight months, although once completed the dog's abilities are far greater than a rat's.
The rats have been living in a cage inside a small prefab building since late 2011, alongside more grandiose buildings and grounds where police dogs and horses are trained.
The rats learn quickly in the darkness the nocturnal rodents prefer.
"They need barely 10 to 15 days to learn to distinguish a certain smell," the policewoman in charge of the project, Monique Hamerslag, told AFP, lifting Derrick out of his large cage and into a smaller one where he will be put to work.
"The more boring a rat's life is, the better" it is at detective work, said Hamerslag, who also discovered that male rats are better at the job than female rats.
Rather shy animals
Four small metal tea diffusers are tied to the side of the cage, one of which contains gunpowder traces. It takes Derrick, a largish brown-black rat, two seconds to point it out with his whiskered snout.
He is rewarded with a 'click' sound and a sunflower seed to nibble on.
Detecting gunpowder traces is a particularly useful ability to have when there are several suspects after a shooting.
Each laboratory test to identify traces of gunpowder is expensive and takes up to two hours, more for multiple suspects.
And while getting fingered by a rat cannot be used in court, it gives forensics a clue as to which clothing they should test first.
Dutch police can only hold someone without evidence for a few hours, so the rats can help prevent a guilty suspect being released.
Hamerslag developed the idea in her thesis while studying to join the police, drawing inspiration from an aid group that used rats to sniff out anti-personnel mines in Tanzania.
Tests on the rat detectives' viability are promising. Wiebes hopes they can start sniffing out real crime by early next year, although he admits that date may be "optimistic".
The major problem with rats is their shyness. They are reluctant to be placed in new situations, such as crime scenes where they might be called upon to find a body in a strange building or drugs in a shipping container.
"It's best to bring the smell to the rats and not the other way round," said Hamerslag. "That means we have to take samples and bring them to where the rats live."
Police rats will never replace police dogs, she said, but they have different and complimentary skill sets.
"A dog is a natural predator, and so is not afraid to explore an unfamiliar place," said Wiebes.
"On the other hand a dog couldn't live like a rat, spending their whole time in a small enclosed space, he'd get bored."