It's not a fuse holder - I've popped off the cap and it's just a two pin female plug. It was a demonstrator so had no mods to it before I bought it (so no HIDs, extra lights etc etc).
The only suggestion that might be credible (aside from all the funny ones!) is the LPG diagnostics port idea. The plug/lead look like they're factory but sort of like an add on. There's nothing like it under the bonnet of my SV6 so nothing to compare with...