On my first playthrough I felt it was far more likely to be either of these things:
1. Investigations related to his citizenship application, since investigations of an elite foreign operative (such as Delta Force) would be far more stringent than a normal application, esp. with him quitting the way he did. That's extremely rare for elite soldiers.
2. Els' Dad checking into him, as it's obvious that he's close to his daughter, and the family is getting involved in their charity venture.
3. Old chickens coming home to roost. Something/someone from his military past ("dead" friend or not). Heck, it could even be related to Alistair or his mother.
I've seen nothing to change my mind. Yeah, the investigators seem sketchy, possibly government or military, but that can be private as well. Yes, there was passing resemblance to some faces we've seen in flashbacks. There was enough of a scope to the investigation that it triggered alerts to the CIA.
Els does not seem like someone to trust an outside team of investigators as much as her own judgement, but I could be wrong. Serena seems more likely to do that, but I don't see any motivation for her to do that. Davis seems more likely as a trigger for the investigation (his death triggered an investigation into possible causes, and his relationship with Val is an issue, or maybe Davis triggered an investigation for whatever reasons he really wanted to reunite).