Future Perfect progressive
The Spanish Perfect Future progressive is a tense that describes actions in progress in a perfect future, i.e "what will have been happening". It's also used sometimes to speculate about something that "may have been happening in the past".
It uses estar in the Spanish Perfect Future followed by a gerund (equivalent to the English -ing form in Spanish)
For example:
Hoy, a las 3 de la tarde, los diputados habrán estado votando en el parlamento.
Today, at 3 in the afternoon, the representatives of the lower house will have been voting in parliament.
Imagino que ustedes habrán estado estudiando en la biblioteca esta mañana, ¿no?
I imagine you have been studying in the library this morning, right?
