Have you checked in each post if there are any custom fields? Might be there...or, if not:
I would look at the single.php file in your theme.
For reference I am making this decision based solely on the output of the body classes:
<body class="single single-post postid-8894 single-format-standard body_width">Note: single.php may make use of a content template part...it may be within that file. If so, you may see something like this in single.php (example from twentyeleven theme):
<?php get_template_part( 'content-single', get_post_format() ); ?>So the file to look for in that case would be content-single.php
If not that, here is what I would do to hasten this:
Download your current theme folder, all files in it. Use you computer to perform a search of the downloaded folder/files making sure to search within the files for this text.