Sucuri shows the files where these 'hidden' iframes tags are to be found, you have to either remove those pages relating to 404 and or find and remove the iframe tags.
The fact that this iframe tag is outside the closing HTML tag is also suspicious in its own right.
The first thing to ask yourself is, is that iframe tag legit, e.g. you created it and the location it is connecting to is correct (corfuparadise.gr).