When hamlet hears of his father’s ghost, what is his immediate suspicion?

Hamlet thinks that something wrong is going on in Denmark. He thinks there have been criminal offences and dark things going on.

“My father's spirit in arms! all is not well; / I doubt some foul play: would the night were come! / Till then sit still, my soul: foul deeds will rise, / Though all the earth o'erwhelm them, to men's eyes”