Will an adrenal tumor (if it exists) show up in an abdominal ctscan?

Maybe. Not all abdominal CT scans would be done in such a way that tiny things would be found. If the doc consulted with the radiologist & focused their attention on a specific set of possibilities it would have a greater chance of finding something by varying their techniques/preparation. A simple abdominal CT done after trauma may not pick up anything but the large obvious tumor.

Adrenal tumors can be very small but still hormonally active. A negative CT would not rule out adrenal tumor in a person who has other evidence of adrenal malfunction.