You read it on here no doubt, that doesn't make it a fact. Why would they need a dog to sniff out gun powder? The reason the dog was there is that they sometimes send in a sentry dog if they feel the situation is too dangerous for an office. A 'crazy person' is unpredictable so the dog was there as another avenue if required. It seems pretty clear reading his statement that they realised pretty quickly that he wasn't required.
The dog would have been deployed because of the knowledge of arms kept at the farmhouse and because the tragedy had only just happened,for all anyone knew,there could still have been an armed person in and around the property,or someone who'd been responsible for having handled a gun.Zeus was a gun dog.
This was mentioned back in 2011/12 by either Curiousessex or HMEssex,both of who must have got their information from somewhere,maybe local to where they live,in Essex.