Lost macro when saving as Non-Macro Enabled Workbook

Hi All

I have written some macro code in a workbook, I then inadvertently saved the file as .xlsx file and not .xlsm
I don't recall seeing the prompt that says "you must save as macro enabled" (or whatever the message is), but either way I missed it.

When I reopen the .xlsx file the macro is still functioning, but I cannot find the code anywhere!

I have removed all protection from the worksheets and workbook.
The macros window (Alt+F8) shows no macros.
The VBAproject window (Alt+F11) shows no code, in either of the two worksheets or ThisWorkbook, also there are no Modules displayed.

I've tried saving the existing file as a Macro-Enabled workbook (.xlsm) but it acts exactly the same way as the original file I.e. no visible code, but macro functions.

The macro itself runs automatically when a specific cell is changed.

The code must be there somewhere because it's functioning, please help

Many thanks