Maybe it means nothing. Maybe it's as pointless to look for meaning in this as in many things Trump does, doesn't do and says.
Maybe it means he's close to taking a permanent vacation from the job, resigning, retiring, working on his golf game full-time. He doesn't seem to be having fun Presidentin', he's complained about how hard the job is.
Maybe new White House chief of staff General Kelly, highly praised by Republicans and Democrats alike as someone highly skilled in bringer order to chaotic situations, encouraged the Main Agent of Chaos to take a very long vacation, hoping to make the White House as orderly as possible during those 17 days. And maybe Kelly will make the administration ship-shape. Still, when/if Trump returns on August 21, how long could the most stable state of order last?
Mueller is not taking a vacation. He's begun issuing grand jury subpoenas. Talking heads on TV, specialists in such things, are opining that the President has already publicly admitted to obstruction of justice, in an interview with Lester Holt. Prosecutors are saying they've indicted people for much less than what everybody already knows Trump has done. More Congressional Republicans are standing up to Trump. Polls show that his approval rating among his base is finally beginning to erode -- which may be the only reason that more Republican Congresspeople have found the guts to openly talk about what a mess he is. Trump's job can't be getting more fun.
Go ahead, Donald: work on your golf game full-time. Except when you take the time to tell wildly-cheering crowds of sheer idiots about how your Presidency was sabotaged and ended by a witch hunt by the liberal elites.