DUKE  These likelihoods confirm her flight from hence;

Therefore I pray you, stand not to discourse,

But mount you presently, and meet with me

Upon the rising of the mountain foot

That leads toward Mantua, whither they are fled.

Dispatch, sweet gentlemen, and follow me.

[Exit]   (5.2.41-46)

These likelihoods confirm her flight from hence: she’s run away! Silvia’s run away, all the evidence points to it, says her father, the Duke. Therefore—he says to Thurio and Proteus—stand not to discourse. Don’t just stand around talking about it, wondering and opining, wasting time and delaying, but rather mount you presently: get your horses right away, and meet with me upon the rising of the mountain foot that leads toward Mantua, whither they are fled. Let’s rendezvous in the foothills (presumably outside the city walls) on the way to Mantua, just where the road starts to climb. That’s where they’ve gone!  Dispatch, sweet gentlemen, and follow me: hurry! And he appeals to Proteus and Thurio’s self-image as courteous and refined, and their class-consciousness as gentlemen, not least because he’s just dismissed Valentine as a peasant. (He’s ignored Julia altogether, unsurprisingly.) Exit the Duke, probably, in a rage and at a run.