It wasn't necessary for us to be present all the time.
We needn't have been present all the time
Perhaps she didn't know about that.
She may not have known about that.
I'm sure he hurt the muscle while playing.
I guess I must have hurt the muscle while playing.
There's a slight chance that they went away
They might have gone away already.
She is unlikely to oversleep. She is never late.
She can't have overslept. She is never late