Don't worry just yet. Most of the time, this is just extra Radioactive Iodine that gets into the salivary glands. Did they tell her to suck on sour candy to make herself salivate? That's why. It clears out the iodine. Otherwise, they light up like a Christmas tree. Ask the doctor about it and see what he says. He may want to keep an eye on it.
As long as your mom is continuously monitored for recurrence, she should be fine.
Also, make sure to ask if they tested your mom for thyroid antibodies when they took the thyroglobulin test. If she is positive for them, it can affect the test. Most doctor check for them, but since they are so important, you should still make sure.
If you have any other questions about this stuff, feel free to write back. Good luck to you and your mom.