To visit Frankfurt is not a problem. True, the German border guards will be tormented by various questions, such as, why, why? Yes, and with the Greeks in the future there may be a problem, since the visa, placed at the Greek consulate will be opened in Germany. This means that the person conceals real plans to visit the countries of the Schengen zone.

To obtain the next Schengen visa, you will have to keep air tickets and write an explanatory that, like, just killed time between flights, and did not spend all the time in Germany. The visa was repaid by the Frankfurt border detachment, and whether you entered the territory of Greece or not, you will not understand.

The most optimal in this situation is to pick up another flight, or just save the air tickets for later submission to the Greek consulate, with a subsequent trip to Greece, .But, believe me, the German authorities really do not like it when they do not come by their own visa .I have 4-5 Schengen visas in my passport, but when I arrived on a Spanish visa to Germany on the border, they arranged such an interrogation with the provision of all vouchers, proof of material security, return ticket and insurance poles .They quarreled that the insurance covers only 20 .000 EUR, and I flew to a very expensive country and many horrors told .But still missed .

You can leave Frankfurt from the airport. After the tour to Greece, it is desirable that the vouchers for accommodation in the Greek hotel will be preserved, since when applying for a visa to the Greek Consulate, questions may arise at the visa center. And confirmation that you spent more days in Greece, and in Germany were transit, will be vouchers.