Many years ago, I was puzzled when a security expert told me that he not only turned his cell phone off, but also removed its battery, if he wished to ensure his privacy. It seemed to me to be unnecessary, and I thought it showed either a level of ignorance or paranoia on his part.
Well, maybe he was ahead of his time back then, but these days I am often reminded of this man. Modern smart phones blur the distinction between the phone being 'on' and 'off' with their standby function and ability to 'wake up' on an event (most obviously, if you set an alarm on a Blackberry, even if the phone is switched off, it will wake up and switch on when the alarm occurs).
And there are some shadowy programs out there you can load onto a person's phone that will allow it to appear to be off while in reality it is on and connected via a phone call to you, allowing you to surreptitiously remotely monitor what is occurring without the phone owner realizing.
These types of programs have one big weakness, however. If the phone's owner looks at his call detail records and sees long phone calls to/from an unrecognized number that he doesn't remember placing, he'll quickly become suspicious.
So, these days, there is a new way of remotely monitoring a phone – using its data service rather than its voice service. This makes the remote monitoring much harder to detect when you are looking at your monthly phone bill, and also makes it much harder to detect when the phone's owner goes to use their phone.
In addition to outright snooping/spying type programs, there is a growing category of 'ordinary or normal' programs that may choose to make use of the phone's microphone to monitor where you are and what you are doing, without telling you. They do this so as to build up a better understanding of who you are, where you are, what you do, your likes/dislikes, and so on. Needless to say, this information helps them build up a profile about you, and is then used to send advertising to you based on what advertisers think you might be interested in.
These programs can tell if you are alone or with other people, if you are watching television (and what you are watching) and can even tell which stores you visit (some stores now play sub-audible sounds for the programs to detect).
Does this creep you out? It probably should; and even more alarming is the prediction that these types of capabilities and surreptitious snooping will become more widespread in the future. The best way to prevent this is to stridently denounce such activities before they become, by insidious default, universal and 'accepted'.
Here's a good article which explains more about what your phone might be doing without your knowledge and, most of all, without your consent.
One has to wonder how long it will be before phones start making use of their built in cameras to still further monitor us and our activities. The mind boggles.