Starting with Source Insight 4.0, the software performs a cryptographic handshake with the licensing server. Even if you find a valid key (e.g., a disgruntled employee leaked their real purchased key), the moment you type it in, the software checks if that key is already activated elsewhere. If it detects two concurrent activations from different IP addresses (or too many fresh installs), it and locks the software.
If your development machine is on a secure network without internet access, you don't have to stay in trial mode. You can request an offline license file from Source Dynamics. Once received via email, use the Import License File
A Guide to Source Insight License Key: Unlocking the Full Potential of Your Code Editor
Upon successful purchase and registration, the license key will be provided to you via email or made available through your account on the vendor's website.