It's another case of "phantom voltage".
When a wire that is disconnected/floating runs alongside any other wire for a distance, it picks up (via capacitive coupling) some voltage, which can read on a sensitive enough voltmeter such as a DVM (including most cheapies). There is no current behind this voltage, it's more like the "son of voltage" because it will disappear at the slightest demand to do useful work. So an analog meter, the kind whose needle movement is powered by the voltage it is measuring, would not pick it up.
When a 3-way switch is on, the unused traveler "floats" unconnected, as you can imagine in the below drawing if both switches were up or down. That dead traveler will have phantom voltage since it parallels the live traveler.
Try switching that 3-way switch to the OFF position, as illustrated, so one traveler is connected to supply and the other is connected to the bulb filament and then to neutral. The voltage will "magically" go away.
Colors: black=always-hot yellow=travelers red=switched-hot white=neutral
Also, while you have the boxes open, get a 5-pack of colored tape and tag the 3-way travelers, I like yellow. Make both the same color since there is no need to distinguish them. Travelers are on the brass screws, or the screws not marked "common".
Other things to look for, while you're in boxes, is the "tab" on receptacles between the screws (on some, it may be broken off, very important when changing receptacles), and watching out for troublesome backstab connections and converting those to screws.