Is the use of Imparfait the only option here to talk about the future?

Question

Si tu ne la vois pas en revenant, c'est qu’elle ne pouvait plus attendre.

At the time of their conversation, she is still with them. There is a possibility, though, that she may be gone in the future, prompted by impatience.

Although the following two events will both take place in the future, the 2nd is considered to be in the past, at least vis-à-vis the 1st:

1. Si tu ne la vois pas en revenant,

2. c'est qu’elle ne pouvait plus attendre.

I wonder if the Imparfait "pouvait" is the only appropriate tense to use in this instance?

Answer 1

I would venture:

Si tu ne la vois pas en revenant, c'est qu'elle n'aura plus pu attendre.