John Gill Commentary Genesis 38:13

"And it was told Tamar, saying, Behold, thy father-in-law goeth up to Timnah to shear his sheep." — Genesis 38:13 (ASV)

And it was told Tamar by some of her neighbours, or by some of Judah's family; saying, behold, your father in law goes up to Timnath to shear his sheep; which might be told her as an indifferent thing, without any design in it; but she took notice of it, and it gave her an opportunity she wanted.