In interrogative sentences, the future is usually used to express perplexity or doubt: ¿Dónde habré dejado mi abrigo? Where did I leave my coat? Watch this page: http://www.aescribir.com/resources/grammar/70-uses-future-70-usos-del-futuro/

That's right you can use it in past depending of the context, you can see the tense of the other verb next to "habrá" in this case is "Sido" del verbo ser en pasado, sometimes we use it to make a suposition when we aren't sure about something :D Espero haberte ayudado! <- I used the verb Haber in past too.

is exactly as u have said it.. see... "habra" is an expression.. people who use it sometimes use it as a suposition or maybe to make conclusion... for example.. Suposition or when u are asking something that u and anyone know... for example - Quien habra arreglado la casa? or Quien habra cocinado? or when u are in classes and u don't know who of ur classmates have done the homework or the proyect to present to the teacher so u ask... quien habra hecho el proyecto para exponerlo hoy? :O Now as I said.. also it can be to make a conclusion... it usually is used for future things... - Habra que hacer el deber este fin de semana // this is said for example when u have to present a proyect that was sent one month ago but now u have three days... so u conclude with that... - Habra que trabajar para conseguir dinero // that is used for example when u really need money but unfortunately, ur family neither u have money to eat.. and u have to do it.. there is not more exit that do that... it is a certain conclusiion...~!! I hope it can help u.. kkk I am from Ecuador-Guayaquil so, My native language is Spanish... Suerte!!