At a full council meeting in February 2016, a petition against the Civic Centre relocation was presented by Councillor Hall, where she also took part in a debate.
However, a fellow councillor raised concerns the Tory leader was using her position to benefit "business and personal economic interests".
Although full details have yet to be revealed, the council's head lawyer said it the matter was of a "serious nature" and one to be investigated by the police.
A spokesperson for the Harrow UNISON Local Government Branch said: "As a public sector trade union, we expect all Harrow councillors to comply with the council's rules, standards and procedures when representing Harrow residents.
"We do not expect councillors to enable double standards or to float rules as and when it suits them so that they potentially pursue their own personal interests."