>These messages appear while the app is rebuilding. There is no reference to the location generating the message. The only options on the message prompt are "OK" or "Help" - so I guess they are warnings rather than errors, although clicking 'Help' tells me it's Error number 12.
>I'm not getting these messages when running the application.
>
>On looking more closely at the list of variables complained about, I see that they are all indexes of tables present in the data environments of the several forms I have developed. Does that suggest anything?

How do you open your table for those forms? Do you use the DBC or you open them manually. If manually, can you paste those lines here.

Normally, an unknown reference like that is mostly related to arrays where we have to include EXTERNAL ARRAY laArray to avoid the project builder to react like this. However, in this case, it does not seems to be related.