Signal metering on consumer grade receivers is generally, 'un-calibrated' and can be based on any combination of RF signal power and digital error rate. I don't know if your set can detect or indicate a higher signal quality value.
I agree, terminating the unused cable run, or the splitter port is the next step.
The cable from the antenna to the splitter is now high on my list of suspects. If water has ever entered the coax, it's done.
Coax Seal is a good way to keep water from getting into the connectors exposed to weather.