Non-scheduled audits

Answer:

ISO 9001 says nothing about non-scheduled audits. ISO 9001 refers to an audit program that includes frequency, which presupposes the existence of a schedule for audits. However, if a thing is not included in the standard does not imply that it is forbidden. If this type of audits is foreseen in your quality management system, in addition to the scheduled audits, there is no non-conformity. One final note, beware of the message: employees can consider non-scheduled internal audits as a signal of distrust.

The following material will provide you information about internal audits:

ISO 9001 – 13 Steps for ISO 9001 Internal Auditing using ISO 19011 - https://advisera.com/9001academy/knowledgebase/13-steps-for-iso-9001-internal-auditing-using-iso-19011/
Five Main Steps in ISO 9001 Internal Audit - https://advisera.com/9001academy/knowledgebase/five-main-steps-in-iso-9001-internal-audit/
free online training ISO 9001:2015 Foundations Course – https://advisera.com/training/iso-9001-foundations-course/