PvE boost if you're alone in encounter or instance (in quest). Invisible boost to all non-combat skill and invisible boost for all to hit chances, invisible also means if you have 70% to hit to target, but because of boost you have 90%, you'll see 70% near cursor, to not provide player with information about someone's presence in encounter.
Or same boost if there're no players in ~30 hexes, everywhere including towns. You can even craft something that requires 40% science if you have only 20%, if there're no players near workbrench, steal better from NPC, kill NPCs with +20% to hit chance, travel on WM alone a bit better because of +20% outdoorsman.
Also instead of "30 hexes" it can be "no other players in your FoV" or "You're not seen by other players", to have benefit for sneak skill, so until you're seen by someone, Loner trait will work, if you're in someone's FoV, you'll lose benefit. So you need to craft with low science skill when no one sees you at workbrench (or buy cave/base for it). Also it can affect gathering cooldown, so if you're in mine and no one sees you, you'll get less gathering CD on few %.
Btw penalty can be opposite thing, you can have party, but you'll lose -20% to all skills, may be also you'll be more likely to critically miss, and penalty to your AC, if you're seen by someone.
Also it can affect IN and CH (again it won't be shown in SPECIAL), +/- when no one sees you/someone sees you. So when no one in NCR bazaar, you can trade with Sha Enin with 1 CH! But if some guy spots you, Sha Enin will stop talking to you and start talk about mutants and that you need to see doctor because of your appearance.
Most likely "You're not seen by other players" is more suitable, because loners can actually work for some group or with friends, but they act better when no one observes it, of course their victims usually don't count as observers, but in FOnline it's impossible to know who's enemy and who's friend from system's points of view, so every creature with name above, except for GMs and devs, who sees Loner, should make him suffer from penalty of this trait, if Loner isn't seen by anyone, he'll get boost to all skills, +2 or +1 CH and IN for dialogue/other requirements check.