Well, at least that was alluded to since he could summon Toads, though I thought they said Nardo was the first to master the sage techniques, so shouldn't Minato have a bulbous nose or some other frog attributes(other than eyes) like Jiraiya? Though I could be remembering incorrectly, it's been a while.
Also if he has had Sage mode, why the hell hasn't he used it? Even in his fight against Tobi in the past he didn't use it, you would think that would be like step 1 in a fight right? Going full power, especially when you can die at any time. --- You played the flute but no one was dancing You sang a sad song and none of us cried