Along with the above, keep in mind there are several systems that will have a slight draw on the battery. The radio memory, security system (if it has one), also the pcm "keep alive" that stores previously learned information. Being an older couple how long does the vehicle sit ? It could just be that its a long time between being operated and the sum of the " normal" draws on it are pulling it down. The relays are a very good thought also, there could be some hanging. They are fairly easy to check with a vom. If you isolate it down to a specific feed and there is a relay involved, pull it and ohm it out. The schematic is usually printed on the side, to see if its sticking. I've seen some in another career field be hung and acutally release and actually click once removed and tapped. Which is also a good troubleshooting method. If you isolate down to a circuit and there is a relay, try tapping lightly with a screw driver handle to see if the draw goes away. Hope some of this helps.
