Three people, presumably White House staffers, were seen slipping out a side door Thursday with a bust of our 16th President. The odd sight definitely caught the attention of the U.S. Marine standing guard, as his eyes seemed to say what we're all thinking -- where the hell ya going with Lincoln???

Trump, as you know, loves to bring up Abe's name to pat himself on the back ... often saying he's done more for African-Americans than any Prez since Lincoln. It's possible the sculpture is a personal item Trump brought when he moved into the White House.

Of course, Trump and staffers are prepping to move out of 1600 Pennsylvania Ave to make room for President-elect Joe Biden -- but, as far as we know, ya don't get to take art pieces as souvenirs.