To those who wonder why the mansion was abandoned, If you havent finished reading the Hayate no Gotoku! manga. Hisui, Nagi's friend who is also a candidate successor for the Sanzenin inheritance, took over every Sanzenin property while Negi became a mangaka. So probably, the Sanzenin mansion was abandond due to Hisui's recklessness or she just simply lives in some other place like Mykonos or Mikado's Castle. Also yeah, It's very much likely that this is the mansion from Hayate since this is not the first time that Kenjirou Hata has done a crossover from his other works (Read "These two will be married in 100 days")
Edit: Right, seems like "Kaguya" has some connection with the mansion? according to the newest chapter.