Collection of decision

Hi everyone
So I applied for a discretionary leave to remain 1st extention after being granted the visa back in October 2013 due to being married to a British citizen and all my family being here.
I applied for an extension in March 2016
Had my biometrics done in May 2016
Yesterday, received an email from Home Office. Asking me to come to their office collect my decision. Now I know it is a refusal but has anyone else received such letter, Asking them to come and collect their decision. If so, did anyone go there to collect the decision? How does it work? Do they detain you on the spot?
Any advice would be appreciated.
Thanks.
SH

What makes you think that it is a refusal?

When I was given the discretionary leave back in October 2013, the decision was posted to me along with all my documents with a biometric resident permit. Even on this forum, many people are getting their decision (approved ones) by post and it doesn't seem like they have been invited to collect their decision...

Can you think of any reason they refused your application?

Not at all. All my circumstances were same from the time I was granted the previous leave back in October 2013.