Does GBAK store things like connection and transaction numbers etc. A
point I have just thought about is that the simple act of backing up
the database using GBAK would create a DB Connection, and given that
there is a CURRENT_CONNECTION and CURRENT_TRANSACTION variables exist,
I believe they may be stored in the FDB file.

Which means if there is a scheduled backup during the night, the
"checksum" (in whatever form that might be) may change overnight even
if no person fires up your application.

But before any more suggestions are offered, what needs to be answered
is the aim of doing this:

Is it a detection or prevention. Are you just trying to see if someone
has gone into an application overnight, or do you want to prevent
anyone from logging in in the first place.

If it is to prevent people from connecting, heck, switching the
security database should do it.

If it is simply detection mechanism, then you know what the next
question would be. Who and when? In that case, I would be modifying
the application so that when it fires up or closes it creates a record
in a log table. Not bullet proof by any stretch, if the app crashes
then you don't get that record logged, but if you are simply after a
mechanism to catch out an "end user" who doesn't know how to use
Firebird to delete records etc (not telling them their real password
is also effective), then this is probably good enough.

Adam