Secret lot
Secret Lots are lots that are only available to a Sim that has completed a task before they can enter.
The Sims 2: University
Secret Society Club House
Befriend three or more members of the Secret Society to get access to this lot. Identify members by their llama blazers.
Bon Voyage
Find maps to the secret lots by digging or taking tours on vacation.
FreeTime
In the FreeTime expansion there is a secret lot for each hobby.
Pearless Park