Linden Lab Official:Someone with the last name "Tester" left some unwanted objects on my land. Who is this Resident and what's going on?

Accounts with the last name "Tester" or "Unit Tester" are generally used by Linden Lab to perform timing and reliability tests inworld for actions like teleporting and wearing attachments. The statistics collected are used to monitor and improve system performance. "Tester" accounts are almost exclusively part of an automated system, and should not interact with you in any way. They will likewise not respond to chat or IM. If you encounter objects inworld on your land that are owned by an account with the last name Tester, you should feel free to return them.

Whenever possible, Linden Lab tries to direct Tester avatars to sparsely-populated or infrequently-used areas in order to minimize impact on other Residents.

The Who are Lindens? video tutorial may also be helpful to watch, as it mentions Tester accounts.