In some cases, the police may have the authority to seize dash cam footage if it is relevant to an ongoing investigation or if it can be used as evidence in a criminal case. However, whether or not the footage can be used against you depends on the circumstances of the case and the content of the footage. If the footage shows you committing a crime or violating traffic laws, it can be used as evidence against you. However, if the footage shows that you were not at fault in an accident or that you were acting lawfully, it can be used to support your defense.
It is important to note that the legality of seizing dash cam footage varies by jurisdiction, so it is important to understand the laws in your area. In any potential seizure cases the police will need to obtain a warrant, and you never should or need to volunteer your footage if you think it will prove your guilt.