In the same way that we use modal verbs to say how certain we are about things in the present we can also use them to speculate about the past.
Have + past participle (‘have done’, ‘have been’ have stolen’ etc.) is called the perfect infinitive. When we use modal verbs to talk about the present they are followed by an infinitive without ‘to’. When we use modal verbs to talk about the past they are followed by a perfect infinitive.
We use must + perfect infinitive when we feel sure about something in the past.
We use might, may or could with the perfect infinitive to say that we think something was possible but we aren’t sure.
We use can’t + perfect infinitive when we feel sure something didn’t happen in the past.