A while back, I helped with a... ahem... "hands-on enhanced interrogation" of a captured centurion. I managed to break him, and he spilled the beans about Caesar being ill and pretending not to be and threatening anyone who audibly notes it, and also that there's a spai sappin' mah NCR in the base.

So is there anything to follow up with these two? I did some reading and you can treat Caesar in another quest, but what of the spai?

There is a quest to deliver updated encryption keys to various Ranger camps, that's presumably based on the information coerced from the Legion Centurion. I got that quest at Camp Forlorn Hope from the Camp Commander (in a previous play through) ~ in my current play through, I beat the Centurion to death after he gave me some minimal information - telling me that there was a spy ~ unfortunately I didn't have a convenient save so I let it go. The Lt Babe in charge of the interrogation just says, "Haven't you done enough?" and the quest in never triggered.

You can do the spy part too. I can't remember though whether I got that from the officer in charge of the interrogation, or Hsu, or possibly someone else. It involved staking out the control tower, and listening. It led, iirc, to the bomb on the monorail mission. [this is the first playthough, so some of the details aren't sticking with me yet]

Spy be sappin' your monorail.