Why are you so dubious about opening the squid log for read access while squid writes to them? People do this sort of thing all the time. For example may people sit with a xterm open doing nothing but tail -f logfile. Just imagine if it was harmful: "Just a sec I will check the log file for diagnostic messages.Oh oops! I have to rotate the log files/stop then restart the daemon!". Yuk.

If you do start scanning the current log at random intervals starting from the file position that you got up to in the previous scan you will need to take into account the default cron jobs that rotate the logs daily (I think). Have a look at the cron jobs on the machine and (at least on some linux machines) /etc/logrotate.conf.