Haha, I live in apartment on 2nd floor, so I don't exactly have the isolated home issues, but sometimes there are those noises, behind window or in corridor and yep, it's very "WAT *nervous look around*", but probably not nowhere as tense as for you. ^^;
Except the times when in night you can hear that someone upstairs accidentally knocked book off bedside table and it fell on floor/your ceiling... But you figure that only in the morning and can swear it happened next to you in the darkness. ._.
And I'm gonna go now, I'll reply tomorrow. I'm sure it was nothing, that noise that startled you.

Maybe raccoon or something outside.