Adding a block to the page but not declaring it in hook_block_info()
The block may be positioned using the Context module, or via custom code in about a million places. What you can do is temporarily modify the template_preprocess_block() function in block.module, identify the block you're looking for by looking at the data in $variables['block'], and then looking at the output of debug_backtrace() to see the callstack and find the place the block is coming from.