There are no leases...a valid credit card is needed and police remove the rare squatter from commercial properties (just like a hotel). 

Is it necessary to file a police report?

A lot of changes are happening in various areas of the city, the police are patrolling more, neighbors are forming task force to reduce crime, business are moving in & increasing property values.You need to remember, some people live in these areas because that is where they grew up, that is where they feel comfortable with, they have family there, their roots are there,  A lot of these areas have really good people living there who are trying to get by.

I'm a full time police officer living in a triplex.

So very interesting that customer satisfaction is your # 1 goal as this customer is not happy with your company polices or your staff.

Rents are a little low for the area but as for location....it is downtown across from the police station and courthouse and about 2 miles away from the college. 

The quad plex is right next to the police station and within walking distance to two universities.

I have a couple of spots in my lease stating if they get in trouble with neighbors or local police they are subject to eviction.2.)

Make sure she understands if he moves in you will evict and offer to release her from the lease.Your problem is you have absolutely no way of policing the situation or proving conclusively he is there.

You need to be aware of what you're buying seeing it in person.I think you can make some decent money in Detroit because it's becoming revitalized, but you will likely be looking at more of a cash flow play than an appreciation play.The best investors in Detroit are the ones who have the ability to buy into the area because they have the influence to change the area.Dan Gilbert has invested in Detroit, and it's smart for him to do it, because he's literally buying blocks of downtown area, and actually buying his own security force (basically to double a police force) to monitor and control the crime in the areas he buys.If you are just buying one house in an area, you better go check it out.