Volume Down button not working following possible water damage

I dropped my iPhone 6 in the salt water for a few seconds. I immediately turn it off and within 10 minutes had it in bag of rice for about a week. I figured that when I opened to inspect it for damage I would replace the battery, which I successfully did using the iFixit guide. Except for one water damage sensor turning red, there was no other obvious damage or corrosion. I did not completely disassemble the phone. Only went as far as battery removal and replacement. Also blew out the phone with compressed air. Put it back together and it works fine. Just recently however, the volume down button stopped working. Maybe this is due to some corrosion on a connector? The button itself feels normal. I suppose the only fix will be to replace the cable?

I am now wondering if when I opened the phone up to inspect for damage and replace the battery, if I should have done a total dismantling of the phone and checked every nook and cranny for corrosion. Would that be the recommended procedure even if initial inspection reveals no damage?