Can't is often used when we think that something is impossible at the present moment.
"Helen can't be in Spain because I saw her driving past my house this morning."
Can't have + past participle is used when we are sure that something did not happen in the past.
Looking out of your window in the morning, you notice that all the flowers in your garden are missing.
What happened? You take a guess at it:
"A rabbit might have eaten all my flowers."
Let's take a look at that form.
might (modal) + have + eaten (past participle verb)
'It must have been a UFO.'
Imagine that you go to your friend's house, but when you knock on her door there is no answer. You then notice that her car is not there, so the conclusion you come to is that she must have gone out.
