I think a major factor in past years for items "disappearing" from smaller museums, and even some larger state museums, is not the security system, but the people in charge of the museum and how they look at and value the relics in their care. I have seen multiple museums in Indiana and Kentucky lose historic items, not because someone broke in or thwarted a security system, but rather because no one in charge at the museum considered the items of much value, and therefore were lax on handling procedures, protective procedures, and knowing who was "too eager" to help the museum with their non-mainstream items.
Most [not all] cases of museum loss here in the Midwest at county historical societies that I am familiar with have occurred around someone "graciously" offering to assist the museum with identifying items, cataloging items, cleaning items, repairing items, etc., where they were granted easy access with little supervision... often because staff members were eager to get the free help and did not value the items highly [often firearms and related material] and didn't watch what was going on closely. When such "helpers" are given access to the museum's records or record system, he/she can quickly see what is highly vulnerable, what is easily erased from records, what is poorly described, etc., making the "disappearing" game a lot easier.
Major losses in one large museum that I used to visit were actually assisted by an employee of the museum, who was bribed/paid by the thief to help move items out the back door more easily. When finally detected by museum personnel, they faced a BIG problem. Many valuable items that were missing were not owned by the museum; rather, they were on perpetual loan from major old "blue-blood" families who trusted the museum to take good care of their family heirlooms. If charges were brought, publicity would get out about how lax the museum was, and how much "stuff" had simply walked out the back door when no one was watching. It would make the museum look beyond foolish to almost an accessory to the thefts. Worst of all, other remaining valuable and historic items on perpetual loan would be withdrawn from the museum after the news got out, damaging the museum and its reputation. End result: the employee was fired, and no charges were ever brought.
Good security systems are obviously a big help, but from my perspective here in the Midwest, a lot of the museum theft that has taken place in the past as values rocketed on once unheralded items was preventable and due in large part to lax monitoring by museum staffers, poor/lax record keeping, unawareness of increasing market values on all items in their care, and over-eagerness to get "free" or "voluntary" outside help with mundane tasks without closely vetting the individuals or monitoring their work.
Shelby Gallien