If the police want to search your phone for evidence during a criminal investigation, they certainly may start by trying to look on the device itself. But they generally need your consent or a search warrant. This can make it difficult for them to actually look for the evidence they need if you simply do not provide consent and a judge does not authorize a warrant.
But it is important to remember that other parties may also have access to that evidence. Under the third-party doctrine, the police can sometimes approach them to get the same information, without ever unlocking your phone.
Social media messages
One example is if you communicate via social media messages, rather than text messages. Many people do this through apps like WhatsApp, Instagram, Facebook or Reddit. Those companies can also access the records and turn them over to the police. For instance, Facebook was subpoenaed for direct messages in one case and provided the information to the authorities.
Backup programs
Another thing to think about is if you back up your phone using a cloud service. Maybe the police want to look at the pictures and videos that you have taken, for instance. If you regularly back up your phone using Google or Apple, the authorities could theoretically ask them to turn over the information. Investigators do not necessarily need to see the photos on your phone if those are also stored on a cloud server.
Legal defense options
This helps demonstrate some of the complexities surrounding digital evidence. It is very important to know what legal defense options you have when facing charges.