There are multiple ways to install implants.
The most common method is subpectoral which leaves a scar that is usually concealed in the natural fold below the breast, which is what I assume you mean by "tell-tale scarring". I agree she probably did not have that done.
The scarring from transaxillary and periareolar are usually much less obvious. The transaxillary incision can be placed to blend in with existing skin creases in the underarm. The periareolar incision ends up being the boundary between the areola and the normal breast skin. There are even weirder methods.
She has occasionally worn sleeveless clothing, but I have not found any clear photos of shoe's armpits, which seems sort of odd. Arms down, high angle, hair in front of the shoulder, etc. always something blocking the area where transaxillary scarring could be seen.
Periareolar scarring can be nearly impossible to spot without close inspection, but the downsides of this method is that some amount of nerve damage is unavoidable and may impact future ability to breastfeed. For consistent cosmetic results, intentional denervation will leave the areolae and nipple unable to contract/erect... notice how shoe has relaxed perfectly circular areolae and soft nipples in all of the nudes? HMMM.
It's possible she's had multiple procedures and has had both transaxillary and periareolar methods used.
Edit: That absolute dump truck of an ass she's got is a valid point in favor of them possibly being natural, though.