An investment property that has no Jewish tenants, but they know that the owner is Jewish, does the front door require a Mezuzah? 

No, unless the owner stores his belongings there or has an office there. In such a case, the front door and all doors leading to his storage area are obligated in a Mezuzah.

 

Sources:

ע״ז כא, א. שו״ע יו״ד רצא, ב. וראה הג״מ מזוזה ה אות ז. וכבר נתקשו בנימוק״י סוף הלכות מזוזה. וראה בבי׳ מזוזת ביתך להל׳ מזוזה בשעה״צ לשו״ע יו״ד רפו, קמה.