Monitored site that has issue and piwik installation are located at different domains.
Some visitors have their local IP addresses, instead of external ones as it must be.
For example: I saw 192.168.0.24, 192.168.0.69, 10.20.24.2. This occurs for about 5% visitors.
There are normal external IPs for those visitors in Apache logs.
My hosting is not behind proxy server.