The 34-year-old center back last played for Charlton Athletic in the Championship in the 2014-2015 season. Has last cap for the US came two years ago.
However, he was with New York at a practice session on Wednesday.
Onyewu also headed out to a dinner with Assistant Coach Alecko Eskandarian and family as Twice A Cosmo spotted.
TICC reached out to the club and was told that he was just training with them and might return on Thursday, but nothing was expected beyond that.
Cosmos guests at practices have been something of interest to followers of the team due to how often they’re linked to signings.
Georgios Samaras was a potential signing with the Cosmos until his physicals came up short of the club’s approval and went off to sign with Rayo OKC. Niko Kranjčar was similarly “just training” with the club prior to signing for the 2016 Spring season.
More recently, the Cosmos had Kenny Cooper and Andrea Mancini present at their practices. Of the two, Mancini was later signed to a contract.