To quote Roger's Opti review:
"First of all, many of the older Adventurer units had a rather high 'off-state' current drain, of about 600 micro-amps. This meant that storing the unit with the batteries in place for a few weeks could make a bit of a dent in the battery capacity. It was not unknown for people to go to use their Adventurer during a trip only to find that the batteries were flat. It turns out (according to Hydro-Photon, the makers), that the 'off-state' current drain was never meant to be anywhere near that high: it should have been only a tenth of that."
The rub is that "many" which implies "some, but not all." For a specific unit, it's not hard to test for anybody who owns a multimeter and wants definitive data.