A building intended to store goods of any kind would have originally had big doors, like garage doors, so the merchandise could be brought in and out on pallets by forklifts. They would be in the back of the building, and if not, then they had been decommissioned, taken out, and maybe boarded over and covered with stucco. They could be another way in.
If a building was too hard to break into there were other ways. Often the easiest was to take advantage of common failings of human beings — carelessness, laziness, inattention, and impatience. Burglars sometimes came and went unseen because nobody had secured a door, fixed a latch, or gotten up and checked to see what had made a sound. Burglars had gotten into warehouses by climbing into loads that were to be driven inside.