I am not sure how much has changed since 3.36 but in the current version:
Preferences > Miscellaneous > Open calibre configuration folder
Open
viewer-webengine.json in a text editor or a dedicated viewer (
MiTeC's JSON Viewer, free for personal use) and look for
standalone_recently_opened.
Be careful not to edit the JSON file. They're very delicate and will reset themselves if there's any error.
There's also the
\viewer\annots\ subfolder. I think the reading positions re stored here but there's no way I know of to connect them to the books.