Where does this permanent SQLExpress connectionstring come from (not web.config)?

Question

Today I noticed that in my ConfigurationManager.ConnectionStrings the very first instance is .\\SQLEXPRESS . I remembered explicitly removing this entry from my web.config so I checked again, but didn't find a thing. Then I did a search over my entire solution, not a single match.
Where the hell is this connection string coming from and how can I remove it?

Answer 1

It's in there by default, pulled in from your machine config file. To get rid of it just put

<clear />

Above your first connection string to remove it.

Answer 2

It comes from the machine.config.

However you may remove it, if you wish.