European observers saw the satellite and the rocket as a thight pair shortly after launch, during their first pass over Europe (see e.g. here and here and here). While the payload was subsequently observed over more passes, the rocket initially got temporarily lost.
Ted Molczan suggested an alternative search orbit (here) and based on that I made an area search with the Meade ETX-70 on the evening of May 8-9, during 20 minutes (between 20:49 and 21:10 UTC) around the nominal pass time from that estimated orbit, covering a 4 degree wide area centered just south of gamma Umi and 11 Umi. During this search I observed an object of around magnitude +6 passing about 2 degrees south of the predicted trajectory, some 4 minutes late.
This indeed turned out to be the rocket. Independant from me, Bram Dorreman also observed it from Belgium that evening on the same pass, bumping into it by chance during his flash observations. With these observations as the basis for an orbital update, it was found and observed by several observers the following nights. Yesterday, in the evening of May 11-12, I photographed it (see below). During a fine zenith pass near 21:50 UTC it appeared as an easy naked-eye object, reaching mag. +2.5 in the zenith.
In below two (out of a total of 3) images it can be seen ascending in the south-southeast in Virgo, and then crossing Bootes in the second image.
That same night I observed the payload, STSS-ATRR itself, much fainter, with the ETX-70. In addition, I saw Progress-M66 (09-006A), recently released from the ISS, and ISS itself. below is one of the images of Progress-M66, here seen crossing through Leo.