Just the fact that it could be found didn't cross their minds. They were not aware of it at that time.
Nunca supieron de ese hecho.
Nunca se les pasó por la mente buscar, recibir eso.
No estaban conscientes de que eso pudiera ser encontrado, por eso no lo buscaron hasta aquel momento.
Y por lo mismo pensaron después de eso ¿Por qué no? Enviar uno y ver que ...
The sentence is not fully idiomatic in my opinion but the meaning of desconocer is pretty clear: to do not know.
According to the DLE:
2. tr. No conocer.
I've not read the article but the sentence provided means that the couple decided to send the bottle because they have found something amazing, something that they thought was unheard-of.