The night desk, and a visitor after midnight
The night desk is a hotel reception after midnight, often one person and a locked door. Telling it at check-in that you expect a visitor saves time later.
After midnight, a hotel reception becomes a night desk. The lobby is quiet, the main door may lock, and one member of staff handles everything from late check-ins to lost key cards. At airport hotels, where guests come and go through the night, the night desk is used to visitors at unusual hours.
What the desk may ask
Hotels differ. Some leave the front door open all night and ask nothing. Some lock it and open it with a key card or a bell. Some ask that visitors after a certain hour be announced at the desk or accompanied to the room. The only way to know is to ask, and the easy moment to ask is at check-in.
A sentence at check-in
If the hotel asks that visitors be announced, a short sentence when collecting the key is enough: you are expecting a visitor later tonight, and she will ask for your room. Said then, it saves a phone call to the room at the moment she is standing at the entrance.
What to put in the thread
Whether the door stays open, whether there is a bell, whether she should ask at the desk or message you from outside, and whether the lift needs a key card. These details become part of the journey time agreed before the start is fixed, and they make the knock come on time.
If the desk says no
Occasionally a hotel does not allow night visitors. Learned at check-in and passed to the thread straight away, it can be settled before she sets out: the booking is moved or cancelled, and the notice you give decides on the scale what, if anything, is kept. The journal post on the night desk goes further into it.