Propane Smell

This goes back several months, Jan 2016. We store our fiver in our sons pole barn year round except when it is in use. But my grandson called us early on day stating that there was a very strong smell of propane in the shed. The only thing that i could think of was that i might have left one of the propane tanks on. But i also told him to open both overhead doors, Manual opening, Do not turn on any lights, and if they are on, do not turn them off. No sparks needed. He stated that one of the tanks was not turned off tightly. So i guess the task now will be to locate where this gas was leaking from. Our unit is 18 years old, but always kept under cover unless it is on the road and still looks good. In our younger days we took trips lasting 6 to 8 weeks, Camping in Canada, Alaska, Visiting over 20 of our national parks, from Main To California. So i guess i will be looking for this leak? in May when we go to the farm.
Glad we took all of those trips when we were younger.