Data

Your phones are always collecting an insane amount of data on you, whether it be directly or indirectly. An example of indirect data would be data collected from everything you connected to, whether it be mobile networks, wifi networks or even bluetooth devices near you. These can then be used to figure out where you were and what you probably were doing.

That's an extreme simplification of what actually happens, but just be aware that the data collected can be used to connect you to a crime scene. It could potentially also be an alibi, but it wouldn't work on it's own. If you were to try to use it as an alibi, they'll most certainly become suspicious and assume that you've done something with your device.

(For example passing it on to someone else)

Thus the need to learn how plant ideas to make someone believe it's actually their own.

After that, you'll also need to make sure your story matches to a large degree with what your device had been transmitting.