My 2001 Chevy S10 truck battery drains dead after setting not started about a day and a half.5. Used a test light between the neg. post and neg. cable when testing while the positive cable still connected to battery.12. Disconnected the power wire to the fuse box under the hood. Test light goes out. Reconnected power wire light comes back on. Pulled every fuse one by one test light says on. Disconnected each wire plug one by one under and on side of fuss box, test light stays on.

I think after charging the battery get it load tested, you may have proper voltage but a cell could be going bad. You can disconnect the negative cable leave off for couple days that will tell you if the battery could be the problem. There are systems in some vehicles which use voltage all the time but it should not run the battery down in 1-1/2 days.
Over the year some things which has run my battery down. It was not always obvious. 86 blazer the dome light in back was left on. I did not see it in the day but found it when it started getting dark. The door switch shorted and left the interior lights on 72 Chevy wagon. Flood light charger in pickup which was charging 24/7 and draining my battery. After removing the light from the charger had no more problems with battery going dead when not run everyday.
Just a few thought I had when I saw your post.

Have the battery load tested fir bad cell