What was going on in Eldon Square this morning?

Got off the X82 at about half 7 this morning, and basically immediately these sirens start blaring, with the only clear announcement I've ever heard in Eldon Square telling people to evacuate cause of 'an incident'. Basically everyone in the bus station was ignoring it, but the security guards were stopping people from going up the escalators next to Argos, and when I left, a police van came charging down Blackett Street. Was it just a drill or test, or did something actually happen? Can't find owt about it online anywhere, so I'm assuming it was a test, but surely they'd say in the announcement if it was? Been confused all day