robzilla - 5:26 pm on Oct 8, 2013 (gmt 0)
This just means someone (or something, i.e. the bot) has set up his/her/its DNS or local hosts file to point this domain to your IP address. Because the request is for "subdomain.Not-my-domain.com", it will show up as such in your logs. God knows to what purpose, but there you go. It's harmless.