Yes if you use Android, and in particular yes if you've given an app camera access.

Proof: my phone (unrooted, OEM Android) can shoot photos whenever I want it to, even if it stays locked, without anyone noticing. It's a trivial Tasker job I set up myself (think e.g. shooting rear-camera selfies using your smartwatch), but it nicely demonstrates that any app with camera privileges could do that too.