In a three way switch, why would the "open" traveler wire test at 17VAC
It's capacitively picking up some stray voltage from the hot wire that parallels it.  Your DMM has a high-impedance input so there is no place for that induced voltage to go.  If you were to place a load on the wire, the voltage would go to near 0V as you would expect.

Bottom line: It's normal and this is a case where the sensitivity of the instrument you're using (i.e. the DMM) is fooling you.