When Sarah Latham replaced a lost car key with an aftermarket alternative, she was unaware of the openings it would present.

She was recently in a Cairns car park where she unlocked her pink Nissan Micra using the remote key — which also opened a white Micra nearby.

"I was heading back to my car and it unlocked using the remote," she told ABC Radio Brisbane's Katherine Feeney.

"I actually went over to their car and opened their door before I realised that I pressed the lock on my remote and I unlocked her car.

"Had it been someone else that had come forward and realised, they could have taken anything out of her car."

It's all in the frequencies

Ms Latham had lost her keys eight months earlier and replaced them at a local locksmith after considering the dealership alternative too expensive.

Automotive locksmith Chris Rose said the aftermarket key was to blame.

"I've spoken to the suppliers of the device and they have heard of this issue," he said.

Do you know how many cars your key could open? ( ABC News: Gary Rivett )

"When we go into the technicalities of the way this device creates remotes, that's when you begin to realise that there are far less permutations."

Mr Rose said remote car keys relied on emitting frequencies that matched individual vehicles.

"The Nissan remote has thousands of different frequencies," he said.

"That would mean that the likelihood of a genuine Nissan remote opening more than one car are thousands to one.

"The aftermarket device that most of the locksmiths, shoe repair guys and car accessory shops are using, [the frequency] is down in the low tens to one."

Opening garage doors and more

Other motorists told ABC Radio Brisbane that they'd had similar issues.

"So many times, I swore I locked my Micra and I'd return to it unlocked but nothing touched. Even after double-checking, I thought I was becoming very forgetful. Now, I know I wasn't going crazy." — Elisha Wright

"For the longest time my neighbour was wondering who was opening her locked garage door every night. It took a while to work out that after we had recoded our door remotes, we tapped into the same frequency and were opening her door each time we opened ours." — Aly Webster

"This is crazy. I can't believe insurance companies aren't kicking up a massive stink." — Tom Read

"It happens with automatic garage doors also. My neighbour's two doors down garage door used to open whenever we opened ours." — Charlotte Schut

"I've seen an SUV key open a sedan as if it was the exact same key." — Brian Spooner

What about insurance claims?

Paul Turner from RACQ said the company was aware of the issue but no Australian claims had been made against aftermarket remotes resulting in car theft.

One key could open many doors. ( Pexels )

"Firstly, the security we're talking about for opening the door is not the same security for driving the car or starting the cars," Mr Turner said.

"It doesn't mean that just because you can open the car that you'll be able to use ... that cloned fob key to drive the car as well."

He added that aftermarket remotes should not impact insurance claims, but if items from a vehicle were stolen that they would be covered by the owner's contents insurance rather than car insurance.

"It's very common for people to use locksmiths to get back-up keys for their vehicles," he said.

"They're very expensive, those fob keys, if they're the manufacturer's version; thousands of people have done that.

"It certainly does bring home the other issue, which is don't leave things like laptops and valuables really highly visible in your car anyway."