My question asking whether the activity is natural is important. If it's not something that happens naturally then you have reason to look over your shoulder. Take a look at this important article written by Bill Slawski about a patent granted to Google titled, The Google Rank-Modifying Spammers Patent?
The patent is related to signals passed to Google that indicate a web page is being altered in order to influence rankings:
A Google patent granted this week describes a few ways in which the search engine might respond when it believes there’s a possibility that such practices might be taking place on a page...
Those practices, referred to in the patent as “rank-modifying spamming techniques,” may involve techniques such as:
•Meta tags stuffing, and
This is what I was getting at when I posted:
I think links added to existing content that has existed without links may look unnatural.
Now it is confirmed that Google makes a determination of what is statistically within the range of normal. How correct is besides the point. The point is that Google is comparing against what is normal and what is not.
So when you make a big change to your web page, be sure it's a normal change. I try to make my pages good the first time out then don't change them except to remove dead links, including affiliate links.
If most sites within a certain category type
don't change their content, then a site within that certain category type that does change its content in ways that are associated with influencing ranks will stand out. This is what the patent is about, that's what I was referring to. How often does the addition of links to a published web page happen naturally? If not often then you are standing out as trying to spam.
What do I mean by a certain category type? The type of site it is, like a forum, a directory, a WP site, an ecommerce site... For example, in a forum, the addition of a link can be considered to happen naturally as people come along and add to a discussion. As we all know, Google understands when a site is a forum and treats it differently in the SERPs. That's a smoking gun example of how Google understands the context of a site and treats it differently. But what of other contexts? The above patent describes what I was talking about, that Google is measuring what is normal behavior and taking into account what is abnormal.
Does this cover links added to a site as the site expands and grows? Absolutely not. That is normal behavior. Would this cover links added as anchor text to an existing document? I think yes, that would be unnatural.
Getting back to the original post, the answer could depend on how often the addition of links happen to existing web pages within a certain category type, across the entire web. As I asked above: In what context would a link naturally be added to an article after it has been published?