Keys and faces don't really matter. It's a 3-part test.
1. You are a government agent
2. You are on a quest for evidence
3. You are searching in a place where there exists a reasonable expectation of privacy
If all 3 of these are true, you need a warrant (at least in the US). Doesn't matter if the keys, or in this case your face, are right there. The fact that you locked your phone with something the cop doesn't normally have is enough to require a warrant.
If all 3 of these are true, you need a warrant (at least in the US). Doesn't matter if the keys, or in this case your face, are right there. The fact that you locked your phone with something the cop doesn't normally have is enough to require a warrant.