More than any other reason, this is why you should do two things:

1) Secure your lockscreen.
2) Turn on Instant Upload (and set it to upload over WiFi and your Data network) on your Google Account.

These two things would have avoided a good chunk of the hullabaloo here.
* The video evidence would have been automatically uploaded to the cloud.
* The phone would be locked for at least long enough for you to get home and make a copy of the original video.
* You would know if the police had conducted a potentially illegal search of your ('locked container', is the term) phone, if the video (or other data) was missing from the phone when you got it back.

After deadly police beating, witness cellphones confiscated
David Sal Silva was reportedly seen and recorded being beaten to death by California police