Stars coach Luc Eymael recently told this website that Mthembu had extended his stay with the Bethlehem-based outfit.
The striker even represented the club in the Maize Cup final against his former team Kaizer Chiefs at James Motlatsi Stadium in Orkney on Saturday.
However, Eymael now appears resigned to losing Mthembu's services to an Absa Premiership rival.
"Now it seems Mthembu [has] left ... using a false family excuse," Eymael said today. "He seems to be in Cape Town."
According to a source at City, the 30-year-old forward is poised to join the Citizens.
"Mthembu is here in Cape Town, having arrived this morning to sign with Cape Town City.
"There are a few issues to be sorted out, otherwise Cape Town City was supposed to make the announcement this morning."
KickOff.com tried to get hold of both clubs for comment without success.
By Sipho Mlotha and Ernest Fakude