A site I help take care of has just started getting crawled by somebody from 172.31.13.241 and 172.31.42.227 . According to WHOIS info, these are part of a block reserved by IANA for special purposes, specifically private networks.
They're making 400 rpm - while not too bad, and not a problem for the infrastructure, it doesn't entirely feel like benign crawling (e.g. Googlebot will make far fewer requests it seems, and I've previously had other sites crawled at 1 to 5 rpm).
I've banned them for the moment, but would like to know if there is a plausible explanation which points to private IPs in Apache access logs being a normal, "usual" thing for some reason. So far I've been unable to find any such explanations, in Superuser.com, Webmasters SE, or the wider net.
EDIT: Thanks for the comprehensive overview John McNamara. The 2 IPs turned out to belong to the Elastic Load Balancer in front of our AWS instances. The thing is, it usually passes on the IP of the request directly to the instances (e.g. I can see GoogleBot's IP, I can see users' and my own IP while browsing). I used the load balancer's logs to find out the public IP, it turned out to be a product we are trialling out which requires crawling. I believe they usually crawl much bigger websites than ours, hence the unusually high number.