I want to make some changes to error messages, re-directing messages and general text on a site. I am finding it a challenge to locate the file where the value is defined.

Is there a simple way to search for and locate the file or some rule about whether it is within a module folder or elsewhere?

For example, on logout, if I want something different than the standard "Thank you for your visit to our site." What is the easiest way to locate the file containing that text to make a change?

You want to check out the "languages" files...

If you look in the "root" directory of your website you will see a directory labeled "language".
Go in there and go to your language.

Inside your language folder (Mine is "english") you will find a few different files. These files define all those messages you see on your site.

Each file will contain lines that look like

define('_BANNERS_LOGIN_LOGIN', 'Login :');

The first portion is the tag for the thing you are changing.. DO NOT CHANGE THIS PORTION...

You modify the second portion. In this case 'Login :' to say what you want..

You can use a free program such as notepad++ to search the files in the directory to find the messages you are looking for.. MUCH easier then doing it by hand. (Windows program)

Also if the working you want to change is in a module you can find a similar setup inside the module directory for the module in question.

As I look there are 24 files in the core language folder and what you are looking for is in those if it is not in a module.