Well, maybe the transmitter was very, very small. I seem to remember that when Sarah realized it was a transmitter there was a brief flashback and it involved a needle. Maybe the bad guys have a transmitter tiny enough that it can be injected anywhere and does not necesitate surgery.
The wire showed up on an ultrasound. That means it was not microscopic.
A needle insertion would leave a surface mark, wouldn't it? Shove even a thin piano wire into your body and see if it doesn't hurt.
I would think that Sarah has been injured and patched up enough to be suspicious about a lump she just happens to feel in the same spot she has a needle mark??
I guess you could say that she was busy..