Albeit U3 claims no trace of U3 software remains on the host PC it doesn’t hurt to run a registry analyzer package just to make sure. While in the midst of doing this USB gadget research, I found the following free utility.
Ntechnologies has a U3 specific reg analyzer to aid in the registry clean-up process.
Whenever hardware or software is added to a PC there are registry changes. The registry also provides a window into the operation of the OS kernel, exposing runtime information such as performance counters, currently active hardware, software and utilities.
Even using the U3 uninstall software might not leave your PC 100% clean. You can download the Registry Analyzer for U3here.