I have a file that, open opening, always puts up an alert saying "This script cannot be found or has been deleted."

(How I wish the author of that message had been willing to reveal who or what was asking for the script, or what script it was asking for.)

No scripts are defined. I've found no layout with a button that invokes any script. How else can I determine what's causing this message (and fix it!)?

Look in FileMaker Preferences -> Document, there is an option to specify scripts to run at startup and shutdown.

Thank you, Vaughan. That was it. No scripts were defined in the file, but the Document Preferences had a check mark beside <No scripts>. Removing the check mark removed the message about "script not found."