Files locked for editing - is there a concise explanation?

There is a shared spreadsheet saved on SharePoint Online. There are a couple of dozen people who have access to it. Coauthoring is allowed. One person downloaded a copy of the spreadsheet to her desktop and then left the file open while she went to a meeting. Other users were then getting the "file locked for editing" message. I checked the file saved on SharePoint and it was not Checked Out. I couldn't figure out a way around the issue. Based on what I've read reading other people's questions, I believe it would have eventually timed out. But I am confused about why/how it became locked for editing in the first place. I have yet to find a concise explanation of the issue.