I drive a 1991 Toyota Corolla with 65,000 miles. It's a wonderful, reliable car that I have had very few problems with since I bought it new. Over the past year my alternator light has been coming on dimly, but only sometimes. I have had it in to my mechanic several times. Each time the alternator has tested good.
Most of the time when an alternator, charge or battery light comes on dim and/or intermittently, this means the alternator is at fault. There are always exceptions to the rule, and this should always be explained to the customer before a part is replaced in an attempt to repair an intermittent condition.
I'm sure that your mechanic did the routine charging system test procedures on your Toyota. The results were perfect at the time because the problem was intermittent or the results were sub-par and he didn't catch it. At that point he could have suggested the replacement of the alternator on the basis that the chances are very strong this is the problem. The slim chance of the problem being elsewhere in the electrical system should also have been explained to you. This would be your decision, with no guarantee.
Yes, with an intermittent condition such as yours, a component may have to be replaced without a definite confirmation of failure. This is a part of the diagnostic process and would be the safest and most logical path to take.
Right now you're just waiting for the alternator to get bad enough to fail the test. The only problem with this scenario is that your alternator may go from intermittent to inoperable, just like that. Meaning your next trip to the repair shop may be on the back of a tow truck on a dark and rainy night.