That way only your residents will have access, avoiding a LOT of potential problems; you can better regulate the use of the spaces (vehicle parking only; no "living" space; no hazardous material storage); and you can inspect them yearly along with your unit inspections.

I found a post on White Coat Investor about using multiple 401(k)s via a SoloK or some combination thereof with an employer 401(k) to allow one to defer more than $132k (I'm married) into retirement vehicles. 

I don’t want to spend too much time/gas/vehicle-wear-and-tear.

If you do not have plans to purchase more income properties, and are seeking a safe place to hoard your cash, find any other investment vehicle (bonds, mutual funds etc.) that pays more that 3-4% and you will be much farther ahead.

Came by early the next morning on my way to work and when I saw no vehicles in the back yard I went in and saw they had taken off. 

Many use it as a savings vehicle because of the fact that contributions can be distributed penalty free without having touch the earnings.

I think he likes the idea of investing, and is currently exploring real estate as a vehicle to invest, but I'm not sure what end game he hopes to achieve with this investment.

We arrived at this point after MANY wasted hours reviewing video for something that didn’t even happen, or that happened elsewhere (vehicle damage, etc.).

In addition whether this money (100-200k is coming from cash/brokerage account or a SDIRA or other pretax vehicle.  

Looking backwards may not be a great driving technique but it’s a good way to see why the other guy’s investment vehicle burst into flames and try to avoid the same situation yourself.