There is one true test to determine if an investment is passive: once you have made the investment, from that point forward, any activity made by you, regardless of the type, duration, frequency, etc of that action, has no bearing whatsoever on the outcome of the investment.

And how many times would you send out letters to that list and what frequency?

The airways are "owned" by the public, regulated by the FCC and every TV station must apply for license for designated frequencies.

Frequency of mailing?

Also, How many/at what frequency have you done follow ups to your pre-foreclosure.

Poor people are looking for freebees and may increase your frequency of tenants not paying on time.3. 

I'd never had an overweight TSA agent stick his hand in my crotch.Yes, lower frequency does not make it right but the new norm is that the TSA is above 4th Amendment rights.

The cash flow is nice, but the longer I hold the property the lower my average annual ROI is because that initial value creation is stretching over a longer period of time.The other reason is that after 4 to 5 years after a rehab, my properties tend to have a higher frequency of maintenance requests.

Only possibly thing I can think of is that they're compounding the interest at a more aggressive frequency somehow.

Finally, as noted, it also blocks Radio Frequency waves from entering the space.