This morning I released new content that should help you in the event that you are engaged by local law enforcement while on mission. Just pull up the Ingress Information for Law Enforcement page on your device and hand it to them so they can be briefed of your mission.
In many situations, the officer may be intrigued, it is often a good idea to keep a spare activation key available to offer that officer if they so choose to take up the cause.
Also, as we are agents with a cause, we must keep in mind the importance of keeping our communities safe. As you are on mission, always be aware of your surroundings and report any suspicious activity to the proper local law enforcement agency immediately.