A hydrometer test is much better than a load test. Many of the newer testers don't test with a large load. They test with a medium load for a longer time which is not the same and can give false positives. We were actually having troubles with our Ford diesels for a while. One cell of one battery would get weak but only the hydrometer would show it.

You say the battery is suddenly being drained, but is there a chance the alternator is intermittently not charging and the battery is weak when you shut the engine off? Sometimes it's hard to test for a failure when a part is working. Maybe your starter is giving you trouble? Have you checked voltage at the battery when this happens? At the starter? Is it more likely to happen when the engine's hot? After a short or long drive? On humid days? When you buy pistachio ice cream?

If you suspect glow plug relay sticking you could try wiring a light to the glow plugs and leaving it outside the hood in a place where it's visible from outside the car. If you see the light on with the key off you know where to troubleshoot. You might also look around for an audible "low voltage" alarm which could alert you that the problem is occurring in order to do testing.

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