I've been doing a lot of cleanup on my own site for SEO recently.
One of the things I did was to start reviewing my missing pages or what are called 404 pages.
This happens when someone clicks a link that goes to a non-existent page.
Either the page doesn't exist anymore (product deleted, renamed) or someone used the wrong link.
Instead of just showing an error page to people, I started to clean up links and setup redirects so they end up on a helpful page. Usually one similar to what they were looking for or something else helpful.
The first step was to look for which urls were getting a 404 error. I have a special tool for that but here's a way to check for 404 errors on any Shopify store using Google Analytics.
Type in a page into your browser that you know isn't in your store.
Look at the page title in your browser's tab and see if it mentions 404 or Not Found or something unique like that.
Log into your Google Analytics account and go to Behavior, Site Content, and then All Pages.
Change the Primary Dimension to Page Title (screenshot below).
On the right is a search box, enter your page title for your 404 page in here. I used "404 Not found" for mine.
Add a secondary dimension for landing page, this will show the url.
You'll end up with a report like this. Step 4 is in green, 5 is in orange, and 6 is in red.
This will tell you which urls are showing up with a 404 error.
Depending on where the link is located you might need to add a URL redirect in Shopify, create a landing page, or fix your store's theme to use the correct link.
In my case I added a lot of URL redirects to cover some older site migrations. This way the visitor gets to the best page for them and I can still keep all of the SEO benefit from having a link.
All told, it will probably take a minute per url to do this. If you did this review every quarter it won't take that long.
But you'll end up making better use of your existing traffic and get some carryover SEO benefit.
Another simple improvement you can make is to get your structured data in order.
Google has been working on Rich Snippets so I bet there's going to be more search enhancements coming out in 2018 for stores with structured data.
And JSON-LD for SEO is the easiest and safest option in the Shopify app store to automatically add that data.
Already trusted by thousand of stores, it's among the top SEO apps overall.