Hey John!
What's a "hidden trait?"
I know the Sims seem to have "hidden skills" (dancing is the one that comes to mind, there's no "thermometer" meter for it, but they definitely get better at dancing over time).
Like Pam, I've had several Sims, when exposed to something novel for them (like their first visit to a museum, or going to the gym, or a trip to the Day Spa) have the wish later to repeat the activity or "do something" related to the experience. I've noticed they can also "pass this desire" on to folks they converse with (I'll see two of my Sims "Chat" about the pool, and one will have the wish to "Visit the Community Pool," for example.
Even with Sims with no "Traits" biasing them towards an activity (like gardening), I've had several Sims who I've directed to weed or tend the garden, over time, have wishes revolving around gardening ("Find a Seed," "Fertilize Plants," etc.).
I know we can't see the Sims3 Sim's "Memories" (like we could in the Sims2), but I suspect the game IS keeping track of a large amount of data for each Sim from session-to-session, including just some random "Wants." I'm not sure it has anything to do with traits one way or the other, it's just the "want pool" the game populates for the Sim.
-- Happytime