According to Netwerk24, the test was done last October during the Currie Cup.
When Grobler, 22, was informed of the result, he admitted his guilt and made a settlement agreement with the South African Institute for Drug Free Sport (SAIDS). He therefore gave up the right to have a hearing.
In recent press releases, WP Rugby had cited "personal medical reasons" for why Grobler was not in action.
When Sport24 contacted WPRU on Tuesday morning, communications manager Howard Kahn confirmed the union had "no comment on the matter".
The Stormers play their final Super Rugby warm-up match against the Boland Cavaliers at Cape Town Stadium on Saturday, having already notched pre-season wins over the SWD Eagles (34-3) and the Cheetahs (39-31).
They start their Super Rugby campaign against the Bulls in Pretoria on February 14.