Likely Butyric acid, common in rubberized plastics. Could be another off-gas, most likely from recycled plastic from China. China until recently bought our very worst plastics for recycling. Butyric acid and other compounds associated with recycled plastics is the result. Off-gassing of plastics generally is well known and we've been combatting it. But low grade recyclables are particularly bad. Lots of air circulation and odor absorbers are the answer. It will continue until the off gassing stops, maybe never.