There aren't any hints to Petunia showing any signs of magic JK always gives a clue so they we can guess. Yes there is more to Petunia than meets the eye. Seems more like "She's not a squib but she does know quite a bit about the Wizarding world."

Considering that a large majority of the characters are wizards it isn't exactly a tip off that Petunia could be a witch.

How about Filch? Even thought the kwikspell course didn't exactly help he might be able to pull off these feat, finally getting rid of some of that bitterness towards the Hogwarts Students.