The lack of proof that the gun was linked to him sounded quite alarming. What I suspect happened was like the #twitterjoketrial: once anti-terrorism police had got involved, it was impossible to de-escalate it and admit it was just a homeless guy and not a secret terrorist bunker.
(The setup is superficially very similar to "stay behind" operations, so I can sort of see how someone might have panicked)
It's basically "We don't know what this, so let's call it terrorism" - which apparently includes any form of social deviance or unsanctioned initiative.
(The setup is superficially very similar to "stay behind" operations, so I can sort of see how someone might have panicked)