Unfortunately, there isn't a black and white answer to this; there are a few different variables so you kinda just have to investigate each situation separately.

I've been ignoring most of them, because when I do investigate, the story is invariably fishy.

If the investigator deems it has merit they will proceed but if not they will drop it.

I am working on a rehab deal now and once that is complete, I should have enough money to buy a small rental property.After talking to some people, listening to podcast, and doing my own investigations online, I have gotten mixed response as to how to purchase my buy and hold property. 

Hire a private investigator to track him down.

If an apt community has a 10% cash-on-cash return I'll investigate further.

I'm in the early stages of investigating a move from the NYC area to California and a change of career to become a real estate broker.

I get 90% success in obtaining phone numbers and sometimes get their email.I do a preliminary investigation for what Zillow says the property is worth and look at the original note value.  

We felt like we knew the area but we are reacquainting ourselves as we investigate properties and make moves in the area (5 buy & hold SFR to date).The key is having a good team in place.

I have been investigating as I said, sounds like you all ready have your feet wet!