The White House doesn't shut down at 5pm. There's whole books, TV series, and movies about the people that are working late every night at the White House. Likely dozens of people are on staff and work shifts that last until midnight every night of the week.

Service staff, including the storied hundred or so White House butlers, work whatever hours are needed. Then you have Secret Service, Kitchen staff, Natalie, house admin staff like the Chief Usher, maybe even NPS staff as the WH grounds are a national park as well, any of whom I would assume might be given access to the secure but unclassified WH wifi and likely a government-issued device. One device with a proxy among all of those people? Entirely unsurprising.

Maybe it's just the printer that Natalie carries around in her purse.