Learn Spanish (Latin America): La huida habría sido rápida y silenciosa. - The escape would have been fast and silent.

In this context, it is a guess about a completed past event. While English often uses "must have been" or "was probably" for this (as in, "The escape must have been fast"), Spanish uses the conditional perfect (habría sido). This is the standard way to express past conjecture or probability about something that already occurred.

Could we use the future perfect (habrá sido) instead of the conditional perfect (habría sido) to guess about the past here?

It depends on when the escape happened. You use the future perfect (habrá sido) to wonder about the recent past, for actions you would normally describe with the present perfect. You use the conditional perfect (habría sido) to wonder about the further past, for actions you would normally describe with the preterite or imperfect. This sentence implies the escape happened further back in time.

Why are rápida and silenciosa used with the verb ser (sido) rather than estar?

We use ser here because we are describing the inherent characteristics of an event. Even though an escape is a temporary action, when you describe the qualities of an event in Spanish (like a party, a meeting, or an escape), you always use ser. Also, notice that rápida and silenciosa end in -a to agree with the feminine noun la huida.