Try running Squid with a bogus httpd_accel_host value. For example:
Does it still work, or do you get error messages?
Disconnect Squid's network connection while your router/switch is diverting traffic to it. Does the network device bypass Squid? How long does it take to notice the problem?
Repeat the same experiment, but this time kill the Squid process instead of unplugging the network cable.
Enable Squid's user-agent log and see if you are intercepting any nonbrowser web traffic.