My wife and were walking our dog approximately 1300 in afternoon. sky had no clouds, very clear. we both observed two contrails. one was a jet liner flying north, the aircraft completely visible, ~ 25k feet in altitude. the other was flying west and moving very, very fast, abnormally fast with no visible aircraft, no refection. it's altitude ~15k feet, lower that the jet. we watched it for approximately 2 minuets and the contrail abruptly stopped. we waited and watched but it never reappeared. we both have a military background and both agreed that whatever it was, that it's speed was remarkable and very odd the way the contrail abruptly stopped.