Voltage on 3way switch, what does it mean.

I needed to replace a dual gang box that had 2 3way switches on it with a blank plate. I knew what one switch controlled, connected common to one traveler leg (red) and all is good. The other switch purpose is unknown. Decided to test for voltage just to see. I have 120V on Red traveler, 40V on white (traveler) and 4V on black (common screw). Newb question, I'm confused, what does 120V and 40V on travelers tell me? And why is there no voltage on common? I get 4v on that common with breaker off and on, so I'm sure that's from proximity to another wire. Thanks in advance!!