Start-up troubles?

Four days ago, I went to fire it up, and the battery unexpectedly drained. Dash and radio came on but didn't have enough to turn it over. Jumped it and it started, but then started getting a key system error warning. The following two days, it was fine. No battery issues, no key warnings. Yesterday, I ran it 5 minutes up the road to get some gas for my lawnmower. Wouldn't turn over leaving the gas pump. Got a jump, but this time it took a longer jump to fire up, and again, began giving me key system error warning. I went right to my local AutoZone as now I'm assuming I have a bad battery. I had the test the battery and alternator, and both came back fine. I drove it that evening with issue. I went to start it today, dead battery again. Again, took a longer jump to fire up. And again, it is now giving a key system error. Something is obviously draining the battery. Could the key system be doing it? Or is the low battery affecting the key system? Any thoughts here would be great because I'm terribly confused?