I've been presuming -- but no actual definitive knowledge -- that this message is "generic". In that the same /install code handles kicking off both an automatic update (partial files to patch the current installation) and full update (where all files have already been replaced, so no file actions need to happen).
And this /install code is just warning "I could not find any automatic update package to process", just in case your expectation was that you should have been about to initiate an automatic update. This warning would alert you to the fact that something was not uploaded correctly, before you try and commence with the update.
But it does seem like there should be some way to definitively indicate "full package file replacement has occurred", versus "the board's files are still of a previous version, and so some kind of automatic update is the only thing we could be about to initiate here". i.e. Files in the board are for a version newer than the phpBB version currently in the database.
And present a warning more suited to each scenario. e.g. If we can tell full file replacement has already happened, then detecting an automatic update in the /install folder would be "the thing that is unexpected", and deserve a warning or full stop error.
And if we can tell that full file replacement hasn't happened (i.e. files still match the database version), then the warning currently being presented would be appropriate if an automatic update package wasn't found. If not simply a full-stop error saying something is wrong with your upload. Since if the files still match the database version, there is no way a "database only" update gets the user where they intended their board to be.