it’ll show on-again-off-again problems like this. it’ll start right up if it doesn’t sit for long, but it’ll be hard to start if it does sit for awhile. Think of it this way – any battery is essentially a slowly leaking bucket. Every time you start the car, you need a certain amount of charge in the battery (water in the bucket). The older the battery, the faster it loses a charge. If it was sitting for awhile, that charge has had time to drain away such that there’s not enough charge left to start the engine (takes a LOT of stored-up charge to turn over an engine). But if you were driving just yesterday, it was just refilled. That could explain why the mechanic can’t duplicate the problem. He’s not going to let it sit out back for two weeks before testing it again. If the battery is more than 5 years old, consider getting a new one just to eliminate that as a possibility. At $100 it’s a relatively cheap test rather than giving multiple hundreds to the mechanic to try to troubleshoot.