In mid 2016, we did not have the issue of google bot crawling /mobile/ or /m/ pages that did not exist; however, at the end of 2016, google bot started trying to crawl /mobile/ and /m/, and since they don't exist on our site, Search Console reports a 404 under the SmartPhone tab.
Here is a QA that I found useful, and it links off to other discussions.
From the discussions I've read, the act of google bot attempting to crawl mobile/ and m/ directories suggest that google bot may not think that the site is completely mobile friendly.
Our site is mobile friendly (at least the Mobile-Friendly Test reports it so); however, out of 9,000 URLs, one page is being reported as not being mobile friendly on the Search Console Mobile Friendly report. In Nov. 2016, we introduced the mobile issue with that one page, and Google started trying accessing mobile/ and m/ in Dec., suggesting that the mobile issue with that page has something to do with why they are trying to access those non existing pages.
We will fix that issue soon, and we believe that Google will then stop trying to access mobile/ and m/ pages.
To answer your question, "why Google is accessing it and what should I do?", Google may be trying to access those non existing pages because your site is not completely mobile friendly. To resolve the issue, you could make your site mobile friendly; or, if it is mobile friendly, make sure there are no mobile issues reported within Search Console. After we fix our issue, we will check if Google is still trying to access mobile/ and m/ or not and post a comment with the results.