My suspicion is that until depserv's question is adjudicated by a judge, then by the Nebraska Supreme Court on appeal, we won't have an answer. I don't think I can afford to volunteer.
Nebraska law just says that a sign must be conspicuously posted (018.04). It does not require postings at each door or even the type of sign. The Nebraska State Patrol (18.05) "strongly suggests standardized signage and placement but states it's recommendations are not part of state law.
For what it is worth, for my interpretation, one sign on a premises "could" be sufficient to get us busted, even with the size and scope of a mall.
Again, a judge would have to answer this to really know.