I've been a federal employee for many years. Here's my take from the places I worked that were worried about knives (i.e. NASA and the FAA).
Basically ANYTHING will get an odd look, even ones under 2.5 inches or there about. I've never had anyone try to measure a knife I was carrying, but I did already have security giving me the evil eye just for having a folder clipped to my pocket (this was Johnson Space Center, and the location I worked was a test and production environment, so a knife wasn't an unexpected item there).
What I carried at JSC, because quite frankly I needed a knife in the job I had, was a Boker Subcom, Swis Army Cadet, and Spyderco Dragonfly (which is great because it locks and is under 2.5 inch). I don't recommend clipping to a pocket though.
My current job with the FAA supposedly has random metal detector searches. I haven't been hit for one. Again, there's a 2.5 inch limit, but I also know people who regularly carry a knife that size clipped to their pockets without any problem. Those people are using an assortment of ones they find at Cabela's or Academy Sports (don't know specific ones).
What I'd recommend. Get a thin Swiss Army 83mm Alox model (either a Cadet or Bantam) and keep in your pocket behind your keys. If you have a watch pocket, these fit nicely. I can fit an Alox Bantam nicely in my wallet coin pocket where it resides at work. If your security seems like they actually don't follow the 2.5 inch rule aggressively (and with layers of interpretation), use a Spyderco Dragonfly or the like clipped or not clipped.
Now if you really have security that follows the letter of the law, a more full sized 2.5 inch blade like the Spyderco Dice or Emerson Snubby would be a good choice.
EDITED TO ADD: A fixed blade, even at 2.5 inches, will probably get you in trouble with almost any federal building/site security. It follows the letter of the law, yes, but it could be added to the interpretation that it falls as a "weapon" even though by knife standards its under the blade length. I wouldn't suggest getting involved in that battle.