syslog – grep tells “Binary file … matches” – how to find the line(s), which contains “binary” bytes?

I have a log file, written by syslog-ng.
The grep tells me “Binary file syslog matches”.
How can I find that line or lines which cause this?
I’ve tried to read it with a simple python program:

with open("syslog","rt",encoding="utf-8") f:

It can read that log with no errors. (even if I put it in a try – expect block)
How can I find that “binary” line?