It sucks and is expensive.

At all times during the Lease Term and throughout any Removal Period, Landlord and Tenant shall, each at its own expense, maintain a policy or policies of comprehensive general liability insurance with respect to the respective activities of each with the premiums thereon fully paid on or before due date..."

. $30-40K is a great starting point but too risky in one of the most expensive RE markets in the world (SoCal). 

In doing so we determined we needed to upgrade the 50amp boxes to 100amp and took on this additional expense.

If there are less expensive parcels to build on, they will probably be built up first.

Reduce expenses.

Or I may get a trust worthy property manager, but that cost almost would lower the long term returns to below the S&P 500 indexes. 

Personally I'd want to know any expenses over a specific dollar amount and would want to know about lease violations, rent that wasn't collected by the 15th, tenant requesting early termination etc. 

Are there any red flags I should be aware of, such as the vacancy expense or NNN costs?

I only found out when one booking came in for one night that's lower than my cleaning fee.