For traffic that does match any configured firewall policy, what is the default action taken by the FortiGate?

The default action is to deny traffic that doesn't match a policy, not allow it.

The default action is to deny traffic, not allow it, and logging is not enabled for the implicit deny.

FortiGate devices operate on an explicit permit principle; if traffic does not match any configured firewall policy, it is silently dropped by the implicit deny rule at the end of the policy list, and no log is generated for this implicit action by default.

While the traffic is blocked, it is not logged by default when dropped by the implicit deny rule; explicit logging requires a deny policy with logging enabled.