Can capsules be seen on an ultrasound? Will they show up on an MRI? Is an MRI necessary to locate capsules?

If the capsules were thin at explantation then there is no evidence to suggest that they would have changed at this point. Removal en bloc is best performed at the same time as explantation but if they are thin and implants intact often the extra morbidity of removal enbloc outweighs the benefits. 

I am not sure why you would need an USS / MRI or what second surgery you are planning. If you have developed lupus etc you should be cautioned against breast implants again. Perhaps you are considering fat grafting / lift? Is the USS / MRI to look at postop swelling / breast changes? Neither will pick up the capsule if it is thin.

I would ask your PS the reasons for the MRI and there may be no harm in getting a second opinion.

Breast capsules do not dissolve and should be removed at the time of explantation to avoid future complications.  They do not show up on any imaging either in most cases.