Whenever you sign in or out or send/receive changes to a teamwork project, ArchiCAD makes a lock file called something like <teamworkprojectname>.LCK. This file will be in the same folder as your plp and it appears only temporarily to prevent multiple team members sending information to the plp at the same time.
ArchiCAD thinks that David is still trying to sign out, so maybe his computer crashed as he was signing out, or he turned off/hibernated his computer before it was finished? Either way if you like you can simply delete the .LCK file.
Only do this in emergencies though as it's obviously not a good practice. Chances are he may not be able to send his latest changes to the model or sign out now. Oh and tell David he needs to be sure the sign out is complete before he turns off his computer!