15 CFR §285.12

1. (a)In addition to regularly scheduled assessments, monitoring visits may be conducted by NVLAP at any time during the accreditation period. They may occur for cause or on a random selection basis. While most monitoring visits will be scheduled in advance with the laboratory, NVLAP may conduct unannounced monitoring visits.
2. (b)The scope of a monitoring visit may range from checking a few designated items to a complete review. The assessors may review nonconformity resolutions, verify reported changes in the laboratory's personnel, facilities or operations, or evaluate proficiency testing activities, when appropriate.