Sound like a short if the second known battery is doing the same.
If you charge the battery with a terminal off and then gently put the terminal back on. Is there a spark that indicates that something is on.
If so then you need to identify what it is that is drawing current by a process of elimination. You may also need to check the earth strap on the engine block from the battery. They do corrode - steel and alloy with current is a bad mixture.
If you have a jumper lead, use the earth cable only from the negative terminal on the battery to another point on the block and see what happens.
If you bought it from a dealer - take it straight back.
Spruce, would it not spark anyway because of the central locking?
It shouldn't if the central locking is either on or off but this is a good place to start looking. There will be a minor spark as the electronics get a power supply, but it shouldn't be like a welding machine arc at the electrode.
The first place to start looking is something that isn't ignition related as the ignition switch 'deadens'/switches off most items, ie screen heaters, wipers, electric window motors, etc. So a central locking solenoid on the doors is a good place to start as that isn't linked to the ignition. They can sieze up and not switch off.
The radio is another item that may or may not be linked to the ignition and be suspect. The cigarette lighter is another if the lighter hasn't popped out.
The early Ford Transit Connect were notorious for a faulty charge cable from the alternator to the battery. The insulation around the wires seemed to burn leaving the cables exposed. They didn't come in contact with any body work but when they got wet they shorted current to earth.
Again the early Ford Transit Connect suffers from an ecu fault that turns the interior lights on including the cargo bay area.
Switching the lights off at the switch wasn't good enough as the relays were still activated. The short term solution was to remove the relays that are situated in the fuse holder behind the glovebox. The long term solution was to get the ecu software (or is it firmware) updated which Ford refused to do under warranty. They were charging £80 for the electronic upgrade a few years ago.
The trouble is that there aren't fault finding mechanics around much these days - they are parts fitters. The fault finders all work for the recovery firms like the AA, RAC, etc. and they were/are jolly good. When they recovered a broken down vehicle, 9 out of 10 fault observations they reported were correctly diagnosed.
We had a Multispace come into the workshop with water in the drivers footwell. The mechanic who got the job hadn't the faintest idea where to start looking for the problem. He was told to open the bonnet and look for a hole in the box section around the windscreen wiper motor area. His solution was to get onto the technical helpline at Slough and see if anyone else had experienced the problem and what the solution was.
Fortunately for him, it was a grommet that had been ommitted on the assembly line and they had a picture of where the grommet needed to be fitted and the part number of the grommet. That took half an hour. If he had opened the bonnet and looked where he was told he would have seen it in seconds. Then to add insult to injury, they phoned the customer to tell him to hold onto the courtesy car for another day, cancelled the job that the courtesy car was allocated to the following day and ordered the grommet. They had the exact size grommet in a box of assorted grommets in the spares dept, but he was too stupid to ask/look.