Some spy apps can secretly track the location of phone users, see their conversations, access their bank details and send copies of their messages or media to another device.
This can leave the user vulnerable somewhat to being spied on so, if you suspect someone might be tracking your phone, it's worth taking steps to minimise the risk or end the activity entirely.
Experts have identified 10 tell-tale signs which suggest someone is indeed tracking your phone.
These include seeing unfamiliar applications on the device and unexplained behaviour, like the device randomly lighting up.
There are also ways to clear your phone if you believe someone is spying on you.
10-point guide on spotting signs someone may be tracking your every move
1. Screenshots on your phone
If your screenshots appear to have poor quality such as pixilation, someone might be tracking your device.
Experts say these should be fairly simple to recognise.
2. Unfamiliar applications on your phone
Be wary if applications are appearing on your device which you have never seen before.
You can search your phone's storage to find out if there are any apps you do not remember downloading.
These could be disguised as something else. Some to look out for are Kaspersky Safe Kids, Norton Family, Net Nanny and Qustodio.
3. Autocorrect doesn't work properly
Keyloggers are spyware which can allow someone to see what you are typing on your phone.
If you are typing and you notice that the autocorrect is misbehaving, there could be someone watching your messages.
4. Phone needs charging more often than usual
If there are applications continuously running, your phone's battery will drain very quickly, reports MailOnline.
If you notice that your phone is running out of charge at a quicker pace than normal, check if any recently used apps are responsible.
5. Unexplained sent messages
There could be someone accessing your device if your phone is sending text messages that you never wrote or sent. This also applies to received text messages which you were not expecting.
Applications can be used by perpetrators to send commands to the device such as screenshotting, sending emails or taking a picture.
6. Phone is hot
Furthermore, your phone's heat is a clue applications are continually running in the background.
If it becomes got to the touch and you are not using it, there could be someone tracking you.
This is similar to the quickly draining battery point, experts say.
7. Unexplained behaviour like your phone randoming lighting up
If your phone is doing weird things on standby mode like lighting up or pinging without any notifications, there could be some suspicious activity happening.
8. Second-hand phones or those given as presents
Users can download apps which aren't available on the official app store in what is called "jailbraking" or "rooting".
If your phone appears "jailbroken" without your consent, there may be someone up to no good.
If you are suspicious of this, there are apps you can use to check: For Android phones use Root Checker and for iOS devices use Cydia.
If you are buying a second-hand phone, it is advised to restore the device to its original factory settings.
9. Switch your phone off
Spyware apps could stop your phone from turning off so that the perpetrator always has access to it.
These apps could also lead to your phone taking an unusually long amount of time to switch off.
Try switching the device off and on to see if this process works, and looks familiar, experts suggest.
10. Data usage
Your phone's data could be being used if spyware is running in the background.
This is because the person looking at your information is using your phone's connection to send it to them.
On an iPhone, you can check your data usage on settings by clicking on the "Mobile Data" page.
For Android users go to Settings > Network & Internet > Data Usage.
From there it is possible to identify if any of your applications are using an unusually high amount of mobile data.
Experts at Vodafone told MailOnline: "We recommend customers to always exercise caution when accessing links on emails and when browsing the internet.
"We advise to not use “jailbroken” devices as this removes all restrictions on the device imposed by the manufacturer and allows unapproved software to be installed."
