About this scene
Big-box store checkout queue, mid-evening. She's three people ahead in line, in thick black leggings and a loose top, holding a basket with maybe four items. The flash starts at the candy display next to the queue. She turns to see what the noise is and clocks it in under a second. She doesn't move out of line. She rotates her hips just enough to give a clearer angle. The cumshot lands on the back of her thigh. She walks up to the register, pays, and walks out without looking down once.
What to watch for
Watch the rotation. Most reactions in this scenario are full turn-around or fast walk-away. She doesn't do either. She stays facing the cashier and shifts her weight from her left hip to her right hip. That movement opens up exactly one angle and she holds it for the whole twelve seconds it takes him to finish. The cumshot mark is visible on her leggings as she walks out and she has to know it. She doesn't reach back.