Last one

[Teos]

The guard nodded. «Last one back, sir.» He pressed a button to open the second door.

"Last one back, sir" - what does it mean?

Thank you

[DBP]

The guard nodded. «Last one back, sir.» He pressed a button to open the second door.

"Last one back, sir" - what does it mean?

Thank you

It could be the door that he just came thru. In prisons there are a number of doors that people must pass thru. A 'new' can't be opened until the previous one has been closed and secured.

[Temico]

"Last one back, sir"

"Last one back, sir" means "the last one (person) has come back, sir"

For security reasons perhaps, in some places/buildings/etc. there are two doors. The first door can be opened by anyone but the second one cannot be opened without a reason. In your example, the guard is reporting to his superior the reason for opening the second door.