Are you sure you erased that flash drive?
Many people in use flash memory to store data on, for example, USB thumb drives. Non-platter technology based on memory is also used in "solid state disk drives," which appear (1) in ultrabooks such as the MacBook Air and (2) in high-speed drives in some storage arrays (SANS).
If you put confidential information on these types of drives/devices, be aware that it is exceedingly difficult to erase.
In short, things you might think would work to wipe out the data, including multi-pass erase software and even degaussing, can be both very time-consuming and ultimately unsuccessful. If you have a flash-memory-based storage device with confidential information on it that you wish to dispose of, please bring it to ITS Security.
We do not have a recommended mechanism to address the issue at the moment, but we can store the devices securely, and provide for their physical destruction if that's what it takes.
For more information, a paper from UCSD describing attempts to erase such devices in a lab is available here.