I forgot to add some commentary in my earlier post.
There aren't many folders without Axis-locks and Waves that I would trust wholeheartedly. I like the CQC-3Ks because they have a fairly sturdy frame-lock, a Wave, the blade holds a good edge, they are just small enough to be innocuous, but have enough "handle" to use effectively. AND they are inexpensive, which is the real bonus. Best in the sub-$50 market, in my opinion.
Moving on - I recommended mostly fixed blades, because I've honestly found in my experience (academic NPE) that small fixed blades are viewed favorably. Something with nice scales and a leather pocket sheath isn't seen as a "weapon", but as a "tool". If you use it to cut your apple at lunch a couple of days a week for a month or so, no one will ever bat an eye at it again. You also have the advantage of getting more handle length to blade-length. The downside is, of course, that they don't carry quite as easily as a small folder. Simultaneously, you're not worried about lock failure, ever. If you go inexpensive like the Woodswalker, again you lost it, you're out about 30 bucks. If a security guy claims it? You're out 30 bucks. If you have to ditch it in a planter, because you forgot to take it out before the metal detector and lose it? 30 bucks.
Remember that NPE isn't just about "improvised" weapons. It's about misdirection. People will almost never look twice at a knife that looks like something their grandfather or uncle would carry. But sure as your born, a 2.5" fixed blade could ruin someone's day, permanently, even clumsily wielded, and with skill? It's over. But no one needs to know about any skillsets with a blade, unless they become applicable. Or put another way - the best camouflage is to hide in plain sight.