Tests have confirmed the presence of Ibuprofen in a swab taken from Signoff after his win in the Lord Stakes.
Trainer Darren Weir was told last month the post-race sample from Boxing Day showed an irregularity and further tests confirmed the presence of the prohibited substance.
Signoff is one of several horses treated at a Victorian property for tendon injuries to test positive months after their rehabilitation and despite negative results in other samples.
Weir scratched Signoff from a race last Saturday and gave him a hard gallop instead with tests taken after the work-out.
The results of those samples are expected on Thursday.
Racing Victoria stewards are continuing investigations into the erratic results.