Origin and meaning of "The eagle flies at midnight"

Solution 1:

I've never heard it before, certainly not as some kind of common phrase - but it sounds to me like the sort of line you'd hear in a cheesy wartime spy movie. Some sort of code phrase to inform your accomplices of your plans.

Solution 2:

The blues song "Call It Stormy Monday (But Tuesday Is Just as Bad)" has the line "the eagle flies on Friday, and Saturday I go out to play".

Friday was payday for laborers all across the U.S. in 1947 when this song was written. The 50 cent and quarter coins that laborers found in their pay envelopes showed eagle images on their back sides during this era. 1947 was actually the last year that the eagle showed up on the half dollar coin.

See http://en.wikipedia.org/wiki/Walking_Liberty_Half_Dollar and http://en.wikipedia.org/wiki/Standing_Liberty_Quarter

When workers paid for their fun during Friday night parties the evening of payday, you could say that "the eagle was flying" whenever they threw coins to a bartender to pay for a drink.

I wonder if this "eagle flies at midnight" phrase is an adaptation of this expression. If that's the source, this phrase would mean that a lot of money was changing hands at midnight.