First, I'd check in the software's admin interface for anything that looks like it can manage content. Maybe there's an edit button or a widget somewhere that contains that message,

Second, I'd check to see if there is a language file somewhere. Lots of systems are translatable, which means there's usually a file somewhere that manages all the "strings" of the site. Could be accessible via the interface, maybe the file is simply uploaded somewhere in a "lang" folder in "en" or something.

Third, I'd check to see if that software is theme-able, and if so, there might be a theme you can edit somewhere that may have that message hard-coded in there. Even if it isn't theme-able, it might still be built around an internal theme.

Last, but not least, if all else fails, there are really only three other possibilities:

1. The message is in another file on the server somewhere... you can perform a search for that string, looking inside all the files that came with the software.

2. The message is somewhere in the database, meaning you need to search the entire database for that message.

3. The message is being remotely injected. Unlikely, but possible.

~Sovereign