PAGC Chairman Dario Rama said Reyes "has a lot to explain," noting that a Cabinet member receives no more than P50,000 a month, including all allowances.
In his statement of assets and liabilities (SAL), Reyes declared his assets at P4.8 million.
Reyes maintained that he obtained a bank loan for the construction of the mansion at Fort Bonifacio, Makati City.
But the PAGC said that Reyes declared in his SAL that his bank loan stood at only P2.4 million.
The PAGC added it would be forced to complete its lifestyle check on Reyes based on available documents should he fail to present proper papers.
Junior military officers who staged a mutiny in Makati last July 27 have accused Reyes of corruption and allegedly sponsoring terrorist bombings.
He is now facing an investigation after the mutineers implicated him in reports that the government was selling arms and ammunition to the Moro Islamic Liberation Front (MILF).
Early this year, President Arroyo ordered a lifestyle check on "all presidential appointees in the executive branch" from the position of assistant regional director up to Cabinet secretary.
She came to Reyes’ defense recently, after the political opposition called for his ouster along with Philippine National Police (PNP) chief Director General Hermogenes Ebdane Jr.
"I’m keeping my security team intact," she said.
She also said that Reyes’ ouster would weaken her administration.
The President said she has "trust and confidence" in Reyes and Ebdane, who are members of her Cabinet Oversight Committee on Internal Security (COCIS), chaired by Executive Secretary Alberto Romulo.
Reyes earlier told a congressional probe into the mutiny that he did not have any intention of stepping down.
"I will resign if I am the problem. But I am not the problem, I am part of the solution," Reyes said.
"The people who want me to resign… are moving towards a certain objective, they want to reach that objective and I am standing in the way."
"I will stand in their way. I’ll stay here," he said.
Presidential Spokesman Ignacio Bunye earlier shrugged off the political opposition’s demand for Reyes to at least go on leave pending the investigation of the mutiny by a newly created six-man presidential fact-finding commission chaired by retired Supreme Court associate justice Florentino Feliciano.
"That’s the opinion of the opposition but the President has trust and confidence in the people who are in charge of security," Bunye said.
Last February, Bunye said soldiers and policemen may be included in the government’s lifestyle check in the wake of reports that most of the victims of "pyramid scam" operators came from their ranks.
He noted with concern that despite being among the lowest-paid government employees, thousands of soldiers and policemen were swindled through this fraudulent investment scheme.
A "pyramid scam" is an investment swindle in which some early investors are paid off with money put up by later ones in order to encourage more and bigger risks.
Bunye clarified, though, that if ever such lifestyle checks may be subsequently conducted on police and military officials who were swindled of their huge investments, it was not because of these "pyramid scams."
He said the likely inclusion of police and military men in the lifestyle check ordered by Mrs. Arroyo last year on all government officials was the result of an ongoing review by the Palace "to strengthen" the government’s campaign against graft and corruption.
He also said the Palace was then studying a measure to create a special body to assist the PAGC in the conduct of lifestyle checks on government officials, including police and military men.