"yet thy footprints were unseen"

When the waters saw thee, O God, | when the waters saw thee, they were afraid, | yea, the deep trembled.

The clouds poured out water;
the skies gave forth thunder;
thy arrows flashed on every side.

The crash of thy thunder was in the whirlwind;

thy lightnings lighted up the world;

the earth trembled and shook.

Thy way was through the sea,

thy path through the great waters;

yet thy footprints were unseen. 

     Ps 77:16-19 RSV, italics (ועקבותיך לא נדעו) mine. LXX: καὶ τὰ ἴχνη σου οὐ γνωσθήσονται. Vulgate: et vestigia tua non cognoscentur.
     No saccharine "footprints in the sand" for the Psalmist, only a rather fearsome mystery (?).
     And yet the past abscondity of God is neither the immediate context nor the final word.  What is the thrust of this line, precisely?