Which is just as well, because he's likely to come across an awful lot more of them.
According to a recent report, there are an estimated 80million brown rats - or Rattus Norvegicus - in the UK (the smaller black rat, Rattus rattus, which carried the bubonic plague around Europe, is now very rare).
That's more rats than humans - an average of 1.3 rats per person, up by 39 per cent since 2000. And, thanks to council cuts in bin collections, poor sewage maintenance and flooding, their numbers are rising.
Within a year, two rats can become 1,000. The gestation period is just 21 days, the average litter size is ten and, as Rob puts it: "As soon as she's given birth, mother rat's back at it - with her dad, her brother... she's not fussy."
In three months, the babies are fertile, and a rat's life expectancy is about three years.
Multiplying: Cuts in bin collections, poor sewage maintenance and flooding mean rat numbers are rising
Which is one of the reasons why, after the Spinningdale trawler ran aground on the island of St Kilda recently, the Scottish National Trust launched an "emergency rat procedure" - laying traps and bait to protect its rare sea colonies.
According to conservationists, one pregnant female swimming ashore would be enough to decimate the entire bird population. The birds nest on the ground, and rats love eggs.
It's going to be a bit of a battle, as Rob agrees. "Rats are very clever - you've got to know how to think like them," he says.
We've been roaring around Northamptonshire all morning in Rob's huge silver 4x4 - Country music playing, three dogs bouncing about in the back - doing his rounds in local factories, warehouses, pubs and cafes.
Although he's on 24-hour standby for emergencies, the bulk of his and other pest controllers' work is to reduce existing infestations with bait boxes.
The bait - blue waxy cubes laced with poison and threaded on steel rods - is hidden inside black plastic boxes scattered around the premises of Rob's clients.
The idea is the rats pop in, have a nibble, then stagger off to die. Every week, Rob checks the boxes, refills them with bait, removes any dead rats and gives the client an idea on "activity levels".
"A lot of people think we're just out to kill things. But it's all about control - pests are just part of creation in the wrong place. You've got to show respect."
It turns out there are lots of ways to kill a rat.
There's live traps (a baited cage with a door that slams shut once the rat's inside); breakback traps which shut and snap the rat's neck; gas ("for really bad infestations, when they're running all over your feet"); air rifle ("for one-offs") and Rob's trusty terrier Mitch. ("Once, a rat jumped on my shoulder and Mitch nailed it before it hit the ground.")
Rob is less of a fan of sticky pads, which some catchers use, laying them on the ground, trapping the rats as they scamper across.
"I've found a couple of pads with just a tail left on them, and another with a foot. How desperate has an animal got to be to gnaw its own foot off?"
I must have gone a bit pale, because Rob quickly changes the subject - to disease.
While the plague is no longer a concern in the UK, brown rats carry their fair share of disease: thread worm, Weil's disease, cryptosporidium, tuberculosis, salmonella, E.Coli, foot and mouth and toxoplasmosis.
"It's their pee that's the trouble," says Rob. "They've no control over their bladder, so it's everywhere. It can get in through the quick in your nails, or you might rub your eyes. I had toxoplasmosis once, it was like having flu for 11 months."
Another bane of the job is the recent surge in DIY pest control, thanks to the internet.
"Punters buy all the kit but they don't really know rats, and they often can't follow through."
"So they call me. At first it's: 'I don't want to hurt it, take it somewhere and let it go.' But after a while, they just want them dead."
Like poor Jenny, whom we roar round to visit next. When the mouse infestation in her garden wall turned out to be rats, her husband didn't dare tell her.
"Now, I want them gassed, or shot, or poisoned... I don't care. So thank God for Rob."
A big part of Rob's job is calming down his clients.
"A man called once at 5am. He'd found a rat in his kitchen. His wife was asleep upstairs and he was calling from the garden. Me and Mitch sorted it - took it away in a bag, and the wife never woke up."
Discretion is the name of the game. "You can't drive around with a giant rat on the side of your car - no one wants their neighbour to know they've got rats. Though most people seem to know me."
And the biggest he's seen? "Oh, only the size of a small cat." While his residential clients are worried about germs, for his commercial clients, the main problem is damage - millions of pounds worth every year.
Rats can squeeze their body through any space they can get their head through and will eat anything - rubbish, soap, meat, excrement. They'll also chew through anything - coated wire, wood, Tarmac, steel.
But it's more than just hunger. Rats' teeth never stop growing, so if they don't gnaw constantly, their enamel-coated incisors would carry on growing, curve back under their chin, bore into their brains and kill them.
They are also horribly adaptable. A colony was once reported to have been found living in a refrigerated lorry at temperatures of -10c. Their hair had grown to four inches long and they were the size of large kittens.
And rat poisons have to be constantly updated as they develop immunity to them.
After 16 years in the job, Rob has plenty of horror stories.
Like the man who found a rat peering out of his loo - "I've heard of tails tickling people's bottoms."
It hasn't just been rodents. Rob has also come across two dead bodies. "I was looking for dead rats, so it was a terrible shock."
So what are the signs of a rat infestation to look out for?
"Noise, a musty smell and droppings. Other signs are faint smears along the skirting boards."
And as for Rob, does he still jump when they pop out? "Oh yes - they can leap four or five feet."
Our final stop is at Rob's local pub. We start checking the bait boxes. Not a single live rat, but they're definitely here because every last scrap of poisoned wax has gone.
"They've even taken the steel spikes that held the bait," says Rob. "You've got to admire them. Just a shame it's a quiet day. You should have been with me on Monday - they were running all over the shop."