This really looks like some var_dump/print_r output left by one of the previous developers.
You are in quite a bad situation as you don't even know if there are any core hacks. So I think your main talk is not to get any output but to cleanup the installation. And you will definitely solve this "arrays" issue in the process.
I would suggest to start with detecting core hacks. You can find them by referring to "In-Core Modifications" section of my answer here.
Then you can try reverting to default theme and disabling 3rd party modules one-by-one. You will definitely locate your issue in process.
