"Could be" if its got an issue, but normally, no.
Say the radio's memory circuit draws 0.001 Amps (1 milliamp) and you have a fresh 100 Amp Hr. Battery then,
100 Amp Hr/0.001 Amp = 100000 Hrs., ("Amp" cancel)
100000 Hrs/ 24 Hrs/Day = 4166.667 Days, ("Hrs" cancel)
4166.667 Days / 365 Days/Yr = 11.4 Yrs. ("Days" cancel)
You say "everything turned off, you have a "main battery switch" then?
And there are no other wires connected to that battery before the main battery switch? If no wires before a main switch, might be a bad battery.
If perhaps there is a wire or two directly tied to the battery ( I have two), then maybe you have something like a auto bilge float wire w cracked insulation/butt splice connector/s laying in some conductive'ish bilge muck to a grounded alum hull?
At any rate, this "stray" current draw sounds low enough that it can be monitored by a inline current meter(or a clamp type if ya got one) sounds thou, insert the meter at the battery post, start disconnecting things/pulling fuses till the current goes to zero'ish.
And, don't forget to look backwards too, loads are one thing but.........Something in the charging circuit might be heading toward ground drawing current and can become a load as well.
