Plueral effusion mystery

PCP discovered fluid around my husband's left lung in early November
during a routine exam. Since then he's had:

A week of a strong antibiotic in case it was pneumonia
CT Scan - negative
Thorasentesis - all tests on fluid negative
Many chest x-rays
Echo Cardiogram - showed that at some time my husband had had a very
small heart attack. It does not show up on an EKG and did no damage.
Stress test - fine - also shows small heart attack in the past.

He was sent to a pulmonologist who was stumped.

His cardiologist thinks it may have a cardiological basis. He said a
blood vessal in his neck was slightly enlarged and this and the small
heart attack could be affecting the pressure in the heart which could
cause the fluid. He treated him with Lasix for a month but the
effusion remains the same.

The cardio says he's really not concerned since all of the serious
possibilities have been ruled out and thinks that since there are no
bothersome symptoms we can just watch it.

My husband feels fine. No symptoms.

Is it unusual for a plueral effusion to be such a mystery?
.