Hey Steve,

I suspect that time you have published this blog post https://medicalcannabis.com.au/medical-cannabis-feed-blog/police-hand-out-cannabis-scratch-and-sniff-cards-for-drug-awareness-1 in previously when the system detected the blog alias same as the existing blog post, it will append to -1 so it will not store the same blog alias into the database.

It might be Google crawled this blog post on this page last time and cached this into Google webmaster tool, but after that you removed this blog post, Since Googlebot regularly visits your site, then it detected this blog post unable to access now, then it will return 404 error on Google webmaster tool and send a email notification to you regarding this.

By the way, if Google already index your site it means google will save your webpages into their databases, now they regularly check your website in order to update their database. Google does not just rely on your sitemap, once they index your webpages, they start crawling same webpage again and again from their databases, no matter if you change your sitemap or remove all of the links that point to that webpage.

To work around this, perhaps you can try read this Google Remove URLs Tool and see if there a way to request Google to remove this page URL https://medicalcannabis.com.au/medical-cannabis-feed-blog/police-hand-out-cannabis-scratch-and-sniff-cards-for-drug-awareness-1 from their database so next time Google will not crawling to the same webpage again.