The ocean can be calm without becoming harmless.

Flat water is still deep water with the same currents underneath. Calm describes the surface at one moment, not the nature of the thing, and people drown in seas that looked perfectly friendly an hour before.

Apply that where you tend to relax too early: a situation that has gone quiet, a conflict that stopped being spoken about, a health issue that stopped hurting. Quiet is information about the surface. It is not the same as resolved.