A supernatural being, such as a ghost
A person who secretly observes others to obtain information
Sorry fella, I didn't mean to spook you.
And tried to spook my stepfather in the process, only he lost your tail.
"If we left a note for P. Corbin we'd just spook him," Dean said.
Maybe your goal is to spook the neighborhood with your frightful outfit or win the Costume of the Year award at a party.
He approached, yet not close enough to spook her, and trying to sound nonchalant said, "Fancy meeting you here."