Mary Joe,
You hit the nail on the head as to one of the main risks of what happens when a NNN tenant leaves a single tenant property. From personal experience, I can tell you that what happens next isn't always pretty - which I will get to in a moment.
If you are considering a single tenant NNN leased investment you have to evaluate thoroughly the market, the location, the building, the lease, and the credit strength of the tenant - all are important and should line up.
In 2009, I represented a property investor that owned a 35,000 SF free-standing Circuit City, and when CC closed their doors, my partner and I were tasked with securing a replacement tenant. This was a prime location at the entrance to a major mall in the market - however, the market conditions in 2009, the size of the building, and the fact that this was an older prototype building all worked against us. Regarding the prototype, the building itself was older and long and narrow, so for retailers in the same SF range, the building just didn't fit into their prototype design - especially in a down market where there wasn't much expansion happening and retailers could pick and choose the best vacant CC buildings. Today the building has been carved up to two smaller retailers on the front and a large Chinese buffet in the middle section. The back portion of the building is still vacant. This is an extreme example, as most investors starting in single tenant NNN leased investments probably won't be buying 30,000+ SF single tenant buildings, however, it does highlight the importance of always thinking about what if the tenant leaves? Will my building be in demand? Will I be able to replace the tenant with a similar credit tenant?
Here are two articles which sum up better and in more detail anything I could write on the subject:
The Risks and Benefits of Triple Net (NNN) Properties
http://www.creonline.com/risks-and-benefits-of-triple-net-properties.html
Triple Net Leases: Pro's and Con's
http://www.berkeleyinvestment.com/Newsletters/Newsletter%20March03.pdf
Also, here are some brokerage companies that specialize in the sale of NNN lease investments to give you an idea of what's out there:
http://calkain.com/
http://www.stanjohnsonco.com/
http://www.colliersnnn.com/
Lastly, Net Lease Advisor (which is associated with Calkain) is a great resource for credit information on the retail net lease market. Here is the website: http://netleaseadvisor.com/
Good luck!
Damon
p.s. - Sorry, I couldn't figure out how to make the "link" button work for the websites, so you will need to copy and paste.