Why is nginx returning 301 for domains not matching mine?

I'm seeing what looks like scanning traffic in nginx access.log (perhaps looking for an open relay?)

HEAD http://www.sina.com.cn HTTP/1.1" -/- 301 0
GET http://www.qunar.com/ HTTP/1.1" -/- 301 178
GET /w00tw00t.at.blackhats.romanian.anti-sec:) HTTP/1.1" -/- 301 178

I'm not sure why nginx is returning a 301 response code. Is there anything to be worried about here?

I am using a very simple location match (adding some more config details as requested)

server {
  listen 443
  server_name my.hostname
  **some ssl settings***
  **some proxy settings**
location / {
  proxy_pass https://backend_host
  proxy_set_header Host $server_name
  **set some headders***
  **error handling lines**
 }
}

I found this note in the nginx documentation:

"If a location is defined by a prefix string that ends with the slash character, and requests are processed by one of proxy_pass ... In response to a request with URI equal to this string, but without the trailing slash, a permanent redirect with the code 301 will be returned..."

If I use a better location match (not ending in a slash) would that help? Is it worth bothering?

location /my/base_url {
    [ configuration A ]
}