Excel allowing two users to simultaneously access a non-shared workbook?

I think my boss might have just made a bunch of edits to our workbook in Read-Only mode, and not realized it.
This has been happening for several months, sporadically. However, I'd like to parse this thing out for any real issues we might be facing.

So here's how the story goes:

Boss opens up a workbook living on a Network server first thing in the morning, keeps it open all day making edits. Only has it open all day like this once or twice a week.
Meanwhile, I am able (supposedly) to open the workbook, not Read-Only --- but in write-mode! I'm inputting data, saving and closing without an issue.

Near the end of the day, he pops in to ask what happened. He says when he went to save, Excel notified him that it was Read-Only.

Now my questions is, is that possible?
That is, open a spreadsheet in write-mode, only to be later notified by Excel that it's Read-Only?
(Nevermind that he didn't save a copy and instead redid his work from scratch, which didn't make any sense to me)

I've come up with a couple of possible explanations (which might not make any sense, I'm not an Excel wizard):
1. That the Workbook is "Shared" on his instance of Excel, but not mine, somehow ("Remove" box checked in Trust Center)
2. That a remote access User (from our corporate offices in Chicago, IL) could be opening this workbook, affecting it somehow.

Any input would be greatly appreciated.