Apparently pure white tropical Hibiscus are not that common. Here is one I found: http://www.fancyhibiscus.com/byronmetts.htm . Is you white Hibiscus a single or a double? Can you post a photograph with a better view of the flower?
There is a tropical white in the DG Plant Files by the name of Hibiscus “White Kalakaua” but it has a shade of pink to it.
This summer I also purchased a tropical Hibiscus at Lowes by the name of Tropic Escape “Sunrise Mimosa” by Costa Farms. The tropical Hibiscus sold by Lowes are usually used in landscape or topiary and I was hoping that the Hibiscus lived up to the picture on the plant tag. The flowers exceeded my expectations in size and color and remain open for three days. Hibiscus “Sunrise Mimosa” hasn’t set seeds, which is a desirable trait in landscape Hibiscus but not if you want to breed Hibiscus. Attached are two photographs and here is a link to the Costa Farms website.
Your Hibiscus is starting to look like Hibiscus “White Kalakaua”. White Hibiscus can photograph pink or yellow depending on the lighting, time of day or temperature. There is a game show where the contestant can ask the audience to vote on the answer. You can do the same thing in Google Images, now that we have a possible name and a better image of the flower. Here is the search:
Based on the images which were returned, Hibiscus “White Kalakaua” looks like a really strong contender. Hibiscus “White Kalakaua” and Hibiscus “Elephant Ear” are synonyms. This Hibiscus is fast growing and is used in landscaping. See this post.
I did see the photograph you sent me but thought you were going to post a higher resolution image, from your message. While the photograph is low resolution several of the leaves clearly show the halberd leaf conformation (see attached composite photo) typical of Hibiscus laevis syn. militaris. If the flowers are the dark pink color, then there is a very good chance you have the correct Hibiscus. If you return to the location be sure to collect seeds in addition to digging a root. If you have a car navigation system, be sure to record the exact Latitude and Longitude.