The reason for waiting two days to check for voltage is that on the car it drained in two days. Off the car it didn't drain over two days ... ergo not battery but external cause.
The battery does not need reviving with Epsom salts. As the OP states, the battery has held a charge over two days so that is not the problem. Epsom salts are used for desulphurisation. The plates inside the battery get coated and form a barrier between the plate and the electrolyte. The battery then fails to charge correctly and also fails to hold a charge.
A good battery charger, one with several phases of charge, should have different cycles the first of which desulphurises the plates.
Loose battery connections will invariably create difficulty in starting ... which should be obvious to the OP. As starting draws far more current than anything else this is when a poor connection will become apparent. However, you are right that battery connections should always be clean, vaselined up, and tight.
The "normal" drain on a car battery should not be sufficient to flatten it over two days. A trickle charger is a good idea though because allowing a lead acid battery to go flat (like leaving a car standing over winter) is a battery killer.
Thanks for the feedback. If you can get hold of an ammeter and put it in series with the main battery cable (if this becomes difficult you can remove the battery earth and stick it in series with that) then you should see a current being drawn if there is indeed a drain.
Then just pull out every fuse you can find, one by one, remembering to replace it back into the correct position once you have confirmed that the drain current is still there.
Someone watching the ammeter will be a help. Eventually you should find the circuit that is causing the drain.
Just make sure you don't try to start the car with the ammeter in place. BOOM, there goes the ammeter!
Please let us know how you get on.