Locking a Building - Seclusion

I am responsible for the security of an office building. Every day, at the end of business hours, I have to make a round of the building and lock the place. Since there are sometimes employees who wish to continue working, there is a revolving door at the exit that can be used even when the place is locked. I would like to ask: a) Is it permissible for me to lock the building if only one man and one woman remain? b) Should I inform them that they are left alone (some of the employees are Torah observant)?

Shalom!
It is forbidden for you to lock the building when by doing so you are causing those inside to transgress the prohibition of seclusion. Therefore, you must inform those present that they are alone in the building, and you are about to lock it. However, if there is someone who has the key and can access the building during the hours it is locked, there is no issue of seclusion. Therefore, in order to permit them to stay in the building, and to allow you to lock the building, you can tell those remaining that you might return later in the evening. This way, they will not violate the prohibition of seclusion even when they are alone in the building.

Shulchan Aruch, Even HaEzer, Chapter