I can assure you that plastic containers (those that are flexible, that most storage containers are made from.)(not hard containers like yogurt containers) do outgass formaldehyde.
Several years ago, as I started my fabric stash. I stored them in plastic storage containers. I always prewashed my fabric to remove excess dyes. When I opened my container a strong smell was omitted. I immediately developed respiratory distress. My "doctor" said it was formaldehyde outgassing.