OK, I don't know exactly if transparent proxies modify the IP when data passes through-but IF a proxy, then a transparent one, because it seems that he had not to configure one.
It could be even a proxy at the ISP level, like AOL does. In this case only the ISP's proxy IP would be detected if the test does not heed X-Forwarded-for headers.
Anyway, I think it would not hurt to check for these headers IF they are present.