Ooh, tricky one!
Let's go back to our motivations for the rule here, to see if they help.
One reason for asking for notability references is to demonstrate it isn't just a speculative idea of the poster from the equivalent of a drunk discussion in a bar. The problem with those types of questions is:
(a) there is an infinite supply of them. If we accepted them, we would be spending all our effort on inconsequential ideas that no-one but the original theorists care about - and be spending much more effort than the person who thought them up. We would rather focus out efforts on the questions that actually make a difference in the world; ones where many people will be interested in the result.
(b) many of us would like to see the number of wild, unsubstantiated claims in the world reduced, not increased, and offering a forum to share them isn't our goal.
If you saw a sign in a museum, it is clear you aren't suffering from either of those. Depending on the museum and the location, a sign would presumably be seen by at least dozens - maybe hundreds - of people per day. That's tiny compared to some of the claims we see, but I think big enough to deserve a question.
The other major reason for asking for notability is to check the context has been adequately conveyed. What definitions might the original source have been using? Was it intended as a joke, or a real claim? Is this simply a misunderstanding of the claim by the poster, and we are about to attack a strawman*?
So, the question back to you is can you accurately describe the claim and the context? If you question is going to be "And the sign said something along the lines of 'you can get fuel from water'", we probably won't be able to understand the claim well enough to respond.
* Some people have objected to my usage of "strawman" in this sense, but I haven't found a better term yet.
Of course, if the sign is like the one in the Ornithology section of a museum local to me, it will be closed as not a real claim:
Why don't seagulls live near bays?
Because then they would be called 'baygulls'.