Why did Joel fire his gun?

Before crossing the bridge we hear Joel fire his rifle. What would this possibly accomplish other than giving away their position to anyone within ear shot. I am genuinely curious if anyone has a theory on why he would do this that makes sense. The only thing I can think of was he was using it as bait to see if any movement stirred (infected or otherwise) on the other side. Maybe it would work on infected but any good sentry would certainly not giveaway their position. Any ideas?