halacha - Unsupervised meat and closed restaurants/butchers

How does Basar SheNith'alem Min Ha'Ayin (meat that has disappeared from view) apply vis a vis restaurants/delis/supermarkets/butchers that have closed for the night/weekend/holiday? I was just skimming these Halachoth the other day, so I'm no expert - but it seems to me that the Shulḥan 'Aruch, the RaM"A, the Tur and the Nosei Keilim are in general agreement that meat that is left unattended overnight is not permitted to be eaten, but there is some discussion as to whether it may be eaten if it was locked up and clearly marked (i.e. when you get back in the morning you see it in the same place with the markings you left on it, and you can assume it is your Kosher meat and not some other, non-Kosher meat). I believe it is explained that the primary concern is that an animal might have dragged away your Kosher meat and left a non-Kosher piece in its place.

1. Is my understanding correct (it may be overly simplified, but is it generally correct)?


2. How does this play out in a place where there is a large quantity of Kosher meat left overnight in a freezer or meat locker? Can large cuts of beef be left hanging in a large freezer overnight at the butchery or in a large restaurant's kitchen? Assuming the answer is yes, the kitchen/freezer is locked, what about where there is a non-Jewish security guard with a key? Especially assuming that the primary concern is an animal, is there also a concern with trusting the security guard?