Two inputs to an old switch are hot
Here's what happened. 


Your house was built with a wiring shortcut called 'switched receptacles'.  You are intended to install a floor lamp and plug it into that switched receptacle, which is then operated by the switch.  
They used the split-feed feature on the receptacle (breaking off the "tab") to split the 2 sockets, so one is always hot (probably black), and the other is switched hot. (probably red). 
Someone got fed up with this, and installed a ceiling lamp.  
An inexperienced person went through the house and replaced receptacles.  They simply matched up wire for wire.  They have never heard of a "tab".  
The tab was not broken off, so it shorted  always-hot and switched-hot.    
The homeowner didn't care enough to fix it. .  
You show up, unaware the house ever had switched outlets.  You discover always-hot and switched-hot shorted to each other. 


The tab is doing this.  

Your options are to a) find the receptacle that should've had its tab broken off, and break it off; restoring original function.  Or b) cap off red wire at both ends add eliminate use of the switch. 

Certain lights are required by Code.  Switched outlets satisfy this requirement.  So you might need the light.