How do you clean? Is it a damp rag, if so, how damp? Too much moisture in the wrong place will harm more than help. Coils are supposed to be waterproof, but I don't take all such claims for granted. Do you wash the coil under running water and work the cable around? I think you can see what I'm getting at. A coil with trapped moisture won't work right till it dries out inside, but usually it should be fine after drying.
Then there's the coil connector itself. Is it secure?
If you use it in water or mud where it really needs cleaning afterwards, we may be onto something. I don't baby my BH machines and they have fallen over while stood in a corner, or rolled some when II set it down to dig. I wipe off dust with a dry rag or just slightly damp, but I don't fuss over cleanliness. Cleaning it should be fine, unless there's a chance a drip or more happened to get forced inside where it shouldn't be.
If the 505 has a "click switch" for power and sensitivity like my Landstar, the batteries don't need to be removed after every use. Only if it's going to sit for several weeks or months. There is no current drain when off. There's also no real advantage in having the freshest batteries installed. My tests show my several BH detectors exhibit little, if any, loss of performance till the batteries are well on their way down.
My Time Ranger has a constant and slight battery draw even when "off." This is presumably due to some small current provided to the "soft-touch" switches. So on that model, I unhook just one side of the battery terminals. I don't remove them, just swing the battery clips to one side, leaving one post still connected. But even that was hard on the battery wires, so I installed my own power switch.
That should not be an issue with the 505, when it's off, it's off. But my thinking is that all this clipping and unclipping of batteries may be causing undue wear and tear to the clips and wires.
These should be things you can investigate a bit yourself. Beyond these suggestions, it might not hurt to contact BH to see what they think.
Good luck!
-Ed