Would a seroma show on a mammogram or ultrasound?

The mammogram would not be useful in the detection of a seroma. Please note that since Seroma is a collection of fluids in the breasts or under the skin, the breast with it would look larger than its counter-part. Nevertheless, an ultrasound would be able to detect fluids and a seroma depending on the thickness of your breast tissue. The radiologist would have detected something other than a ripple if you had seroma. I hope this is reassuring.Remember to maintain regular follow-up with your surgeon to make sure your breasts are healing well without complications.

I think this is a great question to answer. 
All breast augmentation patients will have either future mammograms or ultrasounds as routine screening on their yearly OBGyn exam. Some patients will need evaluation for breast pain and palpable masses. And some patients will need evaluation after high impact trauma after a car accident. All these scenarios benefit from a different type of exam.
 For the case of a routine yearly exam after the age of 40, a mammogram can still be done to evaluate the breast. It is thought that a mammogram can still see 80-90% of the breast tissue.
For breast pain and a palpable mass, an ultrasound is a good option to rule out a cyst or to evaluate a solid mass for signs of breast cancer. If a cyst is seen, an ultrasound can be used to safely aspirate (remove the fluid with a needle) the cyst. If the mass is solid on ultrasound, a mammogram can be recommended to further evaluate the mass for any signs of malignancy.
In the case of trauma to a gel implant, an MRI is the best method for determining if the implant is ruptured. It is about 90% accurate, but it is the best we have. For the case of a saline implant, the best way to determine rupture is-  time. A ruptured saline implant will either deflate over night or have significant loss of volume in 2 weeks. A slow leak over months can occur, but in my experience is very rare.
Now let's talk specifically about your case. You had palpated a mass on your breasts. Your OBGyn did the correct thing by sending you for a mammogram and an ultrasound. It appears the ultrasound did not show a cyst or a seroma. Again, the ultrasound is probably one of the most specific tests for visualizing a cyst or seroma. It appears the radiologist did not see these findings on the ultrasound. When he examined you, he felt a "wrinkle" from the implant on your chest. All gel implants have some gentle folds and wrinkles. That's what keeps them soft and natural. Thin patients are more likely to feel these wrinkles. It is common for plastic surgeons to have to evaluate patients after mammograms or ultrasounds because folding of the implant was seen. In most of these cases nothing abnormal is seen on physical exam.
My final recommendation is to get a physical exam by a board certified plastic surgeon, and preferably your surgeon if possible. He/she will be able to help you sort through the radiologic exam and physical exam findings.
Again the ultrasound is the best exam for cysts and seromas, so you are on the right track. Close the loop of knowledge by having an exam with a plastic surgeon
Good luck and thanks for sharing your question with everyone!