Instances that fail verification due to "not found" were in the terminated state. The Action could not be verified as a result.
Policies may have Actions that change the state of an asset such as an Azure VM or AWS EC2 instance. Some examples may be to start or stop an instance. Further, a policy action may be configured for Verification. Verification (if enabled in the policy block) runs 40 minutes after the policy is first evaluated. Verification checks the current known state of the asset and reports success or failure based on the current asset state as compared to the expected state set by the action. As an example an instance that is "stopped" by an Action should be in the "stopped" state 40 minutes after the Policy start for that Action.
In some cases we find that an asset may have moved to the "terminated" state between the policy execution start and the verification times. The verification will report "Failed Actions" "Verification complete - Instances not stopped:" followed by the asset identifier, the reason - "not found", and the account identifier for each occurrence.