Must Not Have.......

Must Not Have.......
 

I wanted to say: We must not have noticed that Lilia came home last night. I was unsure how to say it, so I didn't :thinking:

But now that I have had time to think about it, I'm still not totally sure. Are either of these correct?

No nos debemos haber dado cuenta que Lilia volvió a casa anoche.

No nos habremos dado cuenta que Lilia volvió a casa anoche.

The first sentence comes to mind because it is closer to a word-for-word translation, but I THINK I have seen the second construct to mean "must not have," but I'm unsure, so any input with explanations is appreciated.

I don't think that must has the imperative meaning that it usually has in English
in this case.I would translate with probablamente or a lo mejor. There is another way of saying it with the imperfect subjunctive--i believe, but i would leave that solution up to someone with a more perfect command of the language.

You almost got it right.

No debemos habernos dado cuenta de que ...
-or-
No nos debemos haber dado cuenta de que ...

Thanks. I'm glad to know that I was close.

Does that mean that my second option

No nos habremos dado cuenta de que Lilia volvió a casa anoche

is completely off base?

I'm not sure why I thought using the future tense of "haber" was a viable option.

There's no reason to use the future tense in this case, especially to mean what you wrote.

Thank you, Rusty.