An unidentified object

While reducing the remainder of my May 9-10 observations, I found what appears to be a UNID (unidentified object) on one of the images, close to eta Uma:

click image to enlarge

It does not identify with any known catalogued object, or classified object known to us. The appearance is HEO-like (very short trail), it is present on only one (out of a series of five) images taken with the EF 100/2.5 Macro USM, and the trail looks to be part of a flare.

Most likely, it is a tumbling rocket stage of some past HEO launch.