Fellow historians,
A post on /r/WarshipPorn (Here) of the German Pocket battleship Admiral Graff Spee attending the Spithead Naval review on May 20th, 1937 shows the ship with both forward and aft ensigns at what appears to be half mast. I think, due to the lack of sailors around said flags, that the photo was not taken while the flags were either being raised or lowered and the position of the flags is purposeful. German flags at half mast is, in my opinion, consistent with the explosion of Zeppelin Hindenburg two weeks before on the 6th. I cannot find any event between Hindenburg and the review at Spithead two weeks later that would warrant a half-mast order from the Nazi government.
However, it is the only picture I found of Graf Spee at Spithead with the flags in this position. All other images show full-masted flags (example). From the images I have found, it looks like that if the flags were at half, they did not remain in that position for long.
Why are the flags in this particular image in a different position than all the other images taken that day?
Possibly it is due to the Hindenburg airship disaster that was two weeks earlier. I cannot find any good documentation, but there is a picture of flags at half-mast at the DC embassy floating around pinterest http://ghostsofdc.org/2014/01/06/nazi-flag-flies-german-embassy-1937/ and several other low quality sites.
More likely it is merely a "flag dip" caught in progress. Here is an account of the flag dip protocol between merchant ships .
edit: Here is a more detailed description of the navy flag dip
And an account of flag dips by German ship to US ship in 1902