That request is a heartbeat request, not your request. You can tell by this line
[Mar 1 16:44:33.000] Server {0x2b927cbd9440} DEBUG: <URL.cc:1541
(url_describe)> (http) PATH: "synthetic.txt", PATH_LEN: 13,
"synthetic.text" is the magic path used to indicate the heartbeat request.
These happen about every 10 seconds so you need to ignore them. There is a
special check for that request that generates the "Will NOT do cache lookup"
message. Look forward or back in the log file to try to find your request,
which should be easy to find if you know the path or host.
