I have not had my smartphone stolen, but I know several people who have.
In one case, the person was walking in a European city with his suitcase in one hand, holding his phone in the other hand texting with his thumb (i.e., he did not have a tight grip on the phone). Someone just ran by and grabbed it. My friend couldn't run fast while carrying his suitcase, and he didn't want to set the suitcase down to chase the thief.
In another case, someone left her smartphone on a school desk. She wandered away, and when she returned, it was gone.
Here is one other relevant incident not involving smartphone theft: The only time I have ever been pick-pocketed was in Madrid. In preparation for traveling there, I had read warnings about pickpockets, and so I had my passport and wallet in my front pockets and had my hands on top of them. But while I was in a crowded market, someone stole my transit pass from my rear pocket.
These three incidents all highlight the advice in this article [via MA]:
One of the best ways to prevent smartphone theft is to keep an alert hand on the device at all times. Don't loosely hold your smartphone as you lollygag down the sidewalk. Keep a firm grip and keep it close to your body. Better yet, leave it concealed in an interior pocket.
I don't always follow this advice. Often, while returning from theatre or music rehearsals late at night, I walk through one of the dodgier parts of downtown, texting or checking my email. I do keep a firm grip on the phone, I don't "lollygag down this sidewalk" at those times, and I tend to put it in a front pocket at times when I feel wary of my surroundings, but I could probably use more care in those situations.
But here is some additional good advice about things to do now, before the phone is stolen:
In case of theft, you'll want to know your smartphone's serial number and model number. Both are usually listed under the "settings" tab or imprinted on the back of the device. Importantly, smartphones also have a unique device identification number, known as an International Mobile Equipment Identifier (IMEI).
The IMEI is a 15- to 17-digit numerical code assigned to each smartphone by manufacturers. It allows carriers to remotely disable your smartphone when you contact them [source: MPDC].
The carrier can also enter your smartphone's IMEI into a national database that tracks stolen smartphones. All major carriers in the U.S. participate in this database, as do a few international carriers. This database feeds into the Global System for Mobile (GSM) and Universal Mobile Telecommunications Service networks [source: Rouse].
To learn your smartphone's IMEI, dial *#06# and the number will appear on the screen. If your phone is an older model, this may not work. However, you can also turn your phone off, remove the battery and record the IMEI under the label. Be sure to keep the information in a different location than your smartphone [source: Wollan].
You should also install a tracking app. For iPhones, you can try Apple's Find My iPhone. Or, if you have iOS 7 installed on your iPhone, you can remotely launch Activation Lock if your phone is stolen. This technology prevents thieves from erasing your phone's data or disabling location apps. It also prevents thieves from restoring or reactivating your smartphone [source: Friedman]. For Android smartphones, there are several third-party apps including Where's My Droid and Lookout [source: Wollan].
If your smart phone is stolen, call the police and give them the serial and IMEI numbers and any locations identified by your phone's tracking app.