Cadillac 1999 DeVille Battery goes dead after sitting for a few days. Had battery checked and it's good. What should I check?

Yes, if your battery went completly dead that can happen.

have someone tap the starter while you hold the key over to the start position.

Charging voltage should be around 14 volts or more for battery to be properly charged. If significantly below that the alternator is not working properly.

Replace the alternator. Once the battery dies, the alternator should still have a little bit of power left in it, if your car isn't starting after it's been jumped, chances are that the alternator is shorted out. Replace the alternator, and also recharge your battery. Check your battery terminals for signs of corrosion, and clean it off with a scrubbie and a mix of water and baking soda.

you need to do a load test on the battery first to verify its good,charge battery fully,turn headlights on high beam,wait for five min and crank engine with lights on,see if you still have a good cranking time,if you don't replace battery,if you do charge battery up to full again,check voltage at battery post with a multi meter,
should be 13 volts at minimal,now turn headlights on ,heater on,and flasher on and pull in gear,have a friend check voltage at battery to see if it is staying above 12.50 volts,if it is below 12.50 the alternator is not putting out on a load,replace it.if these check out good,check for dome or hood or trunk light not shutting off ,or something else draining battery.

you need to verify the dome lights are turning off before you walk away,something is draining the battery.check you car out after driving it and make sure radio and any other things all shut off.

It doesn't have a short. Shorts blow fuses. It has high parasitic drain. On a cad it might be hard to find but not impossible. Do you own an ammeter?

Replace the brake light switch - sticking . Hope this helps

it isnt making a good conection on the body ground

bad ground to battery main terminal..check and advise