I was walking eastward on Oakwood Ave between Park Pl and South Grove St with my two children in a stroller. This was on the evening of Wednesday August 31st, 2016 at about 7:50pm. My son likes to spot airplanes in the sky and so I saw a commercial airliner flying from west to east across the sky and told him to look. I then noticed that flying next to it was a much smaller craft. This smaller craft was between 1 and 2 commercial airplane widths (wing tip to wing tip) away from the craft and was matching the commercial airliner's speed exactly. Both craft were leaving a vapor trail, although the commercial airliner produced two tails while this smaller object only produced one. The commercial airliner seemed to have a dark blue color while the smaller craft was a color between grey and black. The smaller craft appeared to be about one third of the length of the commercial aircraft (length being from nose to tail). Because of the small size it was difficult to identify a shape to this craft, my best guess is that it was triangular shaped.I watched the two craft fly eastward side by side in a straight line until the trees blocked my view. I don't think my son saw either airplane.My initial thought was, "What the heck is that?". I work in the aerospace industry as an engineer so I began trying to problem solve what I witnessed. My first thought that this was another commercial airliner that was flying at a much higher altitude and it just looked like it was flying next to the commercial craft (which would explain the smaller size). I then thought that this was unlikely because their speeds were matched exactly. The next thing I thought it could be was a fighter jet escorting the commercial airliner. I know this happens occasionally if there is a concern on the commercial airliner. This would explain the greyish-dark color and how to could match the speed of the airliner but also stay so close to it. This could also explain the vapor trail left behind. I was thinking possibly an F-16 fighter jet could explain the small size and single vapor trail.I did not have a smart phone or any camera with me to photograph or film the two aircraft. After I got home and got the kids in bed (about 8:25pm) I Google News search for fighter jets escorting commercial airliners for that evening but didn't see anything reported. I told my wife what I had witnessed and she thought that it was strange as well.