mack - 5:28 pm on Dec 14, 2012 (gmt 0)
These things do happen from time to time, and sometimes it can take a while to discover the source. One example of mines was a site being mentioned (un-linked) in a newsletter with large circulation. It caused a large spike in traffic lasting several days. I eventually discovered the source when It started appearing in serps after various sources published the said newsletter.
Only your raw logs will help establish the possibility of it not being human visitors. What sort of spike are you talking?