Here's a scenario.
Let's say Person 1 makes a web app, distributes it under AGPL v3 and keeps the project updated on their GitHub. Person 2 says hey I want to modify your software and so Person 2 modifies it and puts it on their GitHub as well, but doesn't disclose the original source of the program (no fork relation, nothing that discloses that it's a modified version of Person 1's web app). Would Person 2's not disclosing the original source be considered a license violation?
Would Person 2's not disclosing the original source be considered a license violation?
Of course not. AGPL licenses (and AGPL or GPL free software) existed before git. And you can distribute their source without github: just put a tarball on some website. Read also about copyright infringement. See the gpl-violations website.
(no fork relation, nothing that discloses that it's a modified version of Person 1's web app).
You should put a proper copyright notice in every source file (in a comment), with a comment telling its AGPLv3+ license. Then the relation between original and modification is documented. Read how to use GNU licenses and about the philosophy of GNU.
BTW, from a legal perspective, if you legally find some source code without any proper license, you are just allowed to read that code, not to compile it or use it on a computer.
Person 2 must keep any existing copyright notices intact, but is not required to add notices about the source of the software. They don't have to link to the original version's website.
The AGPLv3 requires that modified versions are prominently marked as modified (term 5(a)), and that the date of modification is recorded. But again, stating that modifications happened does not require Person 2 to point to the original version.
As long as such a notice of modification has been made (arguably possible simply through the version control history), then no license violation has occurred.
The more general point to keep in mind is that the goal of the GPL/LGPL/AGPL is to ensure software freedom for end users, not to ensure attribution or to ensure that any modifications can be included in the upstream project. That often happens, but that's more a side effect of the license terms.
