My car is a 2003 Diamante. It is well-maintained, no issues of any kind. It often sits unused for a week or more but it always starts immediately and the battery always sounds strong. Until yesterday. I last used it a few days ago, but it hasn't been sitting as long as usual. When I tried to start the battery was absolutely flat, no sign of anything, no click, no interior light, no engine LEDs. I checked the voltage and it was around 3 volts. 

I put the battery on the charger and this morning the car started normally. I now have to decide if the battery is faulty or if something could be draining it. My intention is to let the car sit for another day or two, and see what it does. When reattaching the battery I noticed a fair spark with everything turned off but I suppose that could be just the computers and solenoids resetting themselves. I'm not sure exactly how that works with modern cars. I have no way of checking any drain on the battery other than touching the contact on.

In my distant past experience, batteries tended to fail gradually. They would grow weaker and weaker over time, and hold a charge for shorter and shorter periods. Yet everything technical that I touch these days seems radically different from what I grew up with. So what should I expect from a modern battery? Can it suddenly and completely fail without any prior indication the way mine did yesterday, or is there likely to be something else going on?