header warning refers to non-existant line

I’ve found the following in my php error log:
Warning: Cannot modify header information – headers already sent by (output started at …/wp-includes/themes/mytheme/functions.php:506)…..

That would normally indicate a problem at line 506 in the functions.php file, but there are only 505 lines in it and no blank space or extra line. I checked the file in linux vi, and it showed the ?> at the end of the file colored differently than the rest of the characters, so I replaced them, which changed the color. I’m not sure if the message will reappear because I’m not sure what prompted it to begin with, but I’d like to know what’s up with that behavior.

Thanks