SAN DIEGO -- It is the stuff horror films are made of -- someone watching you, listening through your cell phone.

It happened to a man named "Dirk," who wants to keep his real name private. He is worried about a cyber stalker who is an ex-girlfriend.

It took Dirk a year to figure out that she could hear him wherever he was.

Dirk has his suspicions on how it was done as did the 10News investigations team and San Diego private investigator Dennis Kerwin.

Kerwin, of Fucci Investigations, said the technology to cyber stalk is out there.

"You could use any open cell phone as a listening device," said Kerwin.

As long as the cell phone is with you, you are considered vulnerable.

"You carry a device in your pocket that has an eye and ear," said Dirk.

10News investigators searched online for spy phones and dozens of different products appeared.

For as little as $8 -- according to one eBay posting -- users could monitor text messages and see what numbers are being called.

For just $150, users could even listen in on conversations.

The I-Team tested FlexiSpy, which has software that works on Web-based cell phones like Blackberrys.

Users need to have access to the phone for up to 10 minutes. If they do, they can hid a tap on the cell phone.

It is a quick download that is undetectable by the phone's user.

Once hidden, all the eavesdropper has to do is call the number of the bugged cell phone.

During testing, the team was able to hear phone calls. However, the phone does not even need to be on in order to listen in. The I-Team could hear everything in and around the phone when it appeared to be off during testing.

The I-Team was given an Internet log-in that allowed them to monitor what numbers that phone was calling, e-mails and text messages. They could have changed the settings on the bugged phone if they wanted to.

The software is primarily marketed to parents and suspicious spouses. Believe it or not, it is also completely legal.

"It's free market until they come up with a law that says only the government can use it," said Kerwin.

But these programs are not foolproof.

Like a careless stalker, the technology can give itself away.

A couple indicators that you are being monitored:

Your phone will not completely turn off.

You hear ambient noise.

"If you hear fuzz or a click, there is a good chance someone is trying to intercept your signal or already has," said Kerwin.

Additionally, look for your phone to light up for no reason.

During testing, the test phone did not ring when the eavesdropper called in to spy, but it briefly lit up.

Kerwin said, "It is important that we know about these things."

There are ways to protect yourself, Kerwin said.

Kerwin advises taking the battery out when you go home at night. In meetings, turn the phone off and take the battery out.

If you are using a hands-free device make sure it has a wire as many wireless device are easier to tap. Also, keep your phone on you at all times. It takes just a couple of minutes for someone to install the software.

That is where Dirk said his nightmare began, and he wants to warn others.

"I changed my cell number 4 times in one week," he said.

Dirk said if you think you are just being paranoid, think again. That voice in your head could actually be the eyes and ears in your pocket.

Kerwin said those most at risk are children and employees.

On Friday, tune in to 10News at 6 a.m. as 10News uncovers just how common it is for companies to spy on their employees and shows you how to keep safe in the corporate world.