Schultz: They DID blow up the ammo supply!
Le Beau: No kidding!
Schultz: You all knew it, before it even happened. How? H-how did you know it?
Hogan: Schultz, we know nothing, we see nothing, we say nothing!
Schultz: We know nothing, we see nothing, we say nothing. Very good! I'll have to remember that.
Hogan: [To Schultz about the Medical officer] Schultz, I want you to stall him for three minutes, then the Kommandant can see him.
Schultz: But why?
Hogan: We're trying to save Klink from the Russian Front. And if we save him...
Schultz: He will save me!
Hogan: Right. Now get going, raus, raus!
Schultz: Ja Wohl, herr Kommandant! I mean, Colonel! Oh, I don't know what this war would be without him.
Schultz: [To Hogan about Newkirk in a Gestapo uniform] I must report this. It would be worth my life if I do not report this.
Hogan: It's only until tomorrow. He's gonna take it off again.
Schultz: Aahhh.
Hogan: After he steals the tank...
Schultz: Ohh?
Hogan: from the Panzer division...
Schultz: Ohhh?
Hogan: and brings it here into the barracks.
Schultz: Oh, I see nothing, I was not even here, I did not even get up this morning.
Top Hat, White Tie and Bomb Sights - S1-E10
Burkhalter: Colonel, may I remind you, that we are here on official business?
Klink: Official business, yes. Monkey business, no!
Answer: The core POWs regularly escaped and returned to the prison camp because they made it their mission to conduct espionage and commit sabotage in the surrounding German territory. They also collaborated with different underground resistance groups and used a network of secret tunnels to help prisoners from other POW camps to escape, who then relayed vital information back to the Allied forces. Hogan and his men maintained the illusion that Stalag 13 had never had any prisoners escape in order to avoid their covert operations being shut down. Being that the prison camp is set in Germany during WWII, there were no American embassies.
raywest
Hogan has mentioned to different characters that they are actually stationed at Stalag 13 to help allied soldiers and prisoners from other Stalags to escape Germany.