The WARN Act normally applies to layoffs or plant closings at "single" sites of employment. There are detailed regulations on what constitutes a "single site." These regulations extend the "single site" concept to mobile workers (such as traveling salespersons), who receive work assignments and report to management at a "headquarters." These mobile employees may be covered by WARN under those special circumstances.
In Bader v. Northern Line Layers Inc., the workers were at construction sites in several states. Headquarters was in Billings, MT. The workers argued that because the construction assignments were temporary, and the Billings office handled all accounting, billing, payroll, and other administrative functions, they were actually employed in Billings for WARN purposes. The Ninth Circuit disagreed. The court noted that work assignments were made locally. Most of the employees were not Montana residents. The supervision was located locally, not in Montana, etc.
WARN is tricky and each layoff or shutdown needs to be closely examined in light of the applicable regulations and case law.