You've provided precious little information for us to know what's happening: What it does it mean: "without making any changes [the InDesign file] is somehow altered"? What version of InDesign? What operating system?
I am running OS X 10.11.6 and using InDesign CC 2017.
The problem is with all InDesign files…including a file created 2 hours before the problem started. That file was created and saved, it was the opened and without doing anything to it I was prompted to save it before closing. I have also created new files saved them, closed and upon reopening they also had to be saved before closing.
The files I open are not copies or templates.
This issue did not exist before yesterday. I have uninstalled and reinstalled InDesign to no avail. I have also done the same with the Creative Cloud app.
Are you saving the files locally, on a server or on a system that might 'tag' the files making them read as updated?
Are you opening from an older version into a newer version, such as from 2015 Release to 2017 Release?
That would be first thing to look at.
It is also possible that some links are updated automatically or some font mapping was adjusted, which makes the file different from when you opened it.
If your open, save, close then open that file again, does it come up as 'altered' and needs to be saved?
Check the options button in the dialog box where you choose files. There is a setting for Open Copy that could do what you're seeing.
Another possibility is that the OP opened a template, but until such time that this is clarified all we can do is guess.
One more possibility is you exported from it. That could cause a need to save the file.
Again, waiting for the OP to explain more.