When closing the Net Express IDE a message box is displayed for the following error message:

     "X016: Undefined query identifier or location not open"

This follows another message for "The SQL option XBD local server option is running. Would you like to stop the server" to which I click on the "YES" button. 

What does this X016 error mean and how can I get rid of it?

This error message is simply an annoyance and there is a simple way to get rid of it by uninstalling the XDB component of Net Express, if you are not using it.

Go to Control Panel > Add Remove Programs and click on Net Express 5.1 and select Change.

When it asks what you would like to do select Modify.

When the Net Express install screen shows you a list of components, click on the box to the left of XDB Relational Database and select the last option which is to not install the component. It should show with a red X through it.

Click next and finish up the install and XDB will be removed as an option and the error message will no longer occur.