During the first scene at the villa, a person can be seen sitting just inside an archway as the camera follows Sam to the table. The villa is supposed to be empty aside from Sam, Michael, and the two captives.
Michael states that he is pumping liquid CO2 (Carbon Dioxide) into the burn safe to freeze it. CO2 does not exist as a liquid, except at very high pressure. Pumping gaseous CO2 into the safe would work, but would vent large quantities of white gas from the top of the safe as it it being frozen.
When Michael and Fiona crack safe with money and passports, Milovan's Serbian passport is clearly visible. It's not a document issued by Serbian authorities (nor even a good, fake one), due to the fact that alphabet used in passport is not a real Serbian Cyrillic, but Russian Cyrillic, among other big visual differences). Accordingly, at the top of first page title says "Respublic Serbia", but in real passport there is no such title.