I hadn't heard the term "protective press pool" but apparently they follow Presidents around all the time so that the media don't miss major stories (i.e., protecting the media, not the President). Presidents almost never ditch the protective press pool, but apparently Trump did tonight, for reasons unknown:

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Something bizarre happened after the Christmas tree lighting ceremony at the White House
Carlos Garcia - 11-28-2018

President Donald Trump abandoned his protective press pool after the Christmas lighting ceremony at the White House and sped away in his motorcade, according to social media reports.

Anita Kumar, the White House correspondent for McClatchy, tweeted about being left behind in the protective pool and how unprecedented and bizarre it is.

“I am the White House print pooler tonight. That means I am supposed to be with [President Donald Trump],” she tweeted. “Instead, I and the rest of the pool, were left at an event while POTUS returned to the White House.”

“We have been told POTUS is back at the White House but technically we don’t know that because we were not with him and did not seem him,” she added.

“I have been a print pooler for six years during the Obama and Trump administrations,” she explained. “I have never been part of a protective pool that was left behind until today.”

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https://www.theblaze.com/news/2018/11/28/something-bizarre-happened-after-the-christmas-tree-lighting-ceremony-at-the-white-house