yes. batts lose their charge over time even when not used.
it varies depending upon the particular battery type/technology. obviously, non-rechargeable batts are more stable (Li being perhaps the most stable popular consumer batt technology currently in use) than rechargeable batts. amongst rechargeables, the rate at which charge is lost varies once again based upon batt type/technology (i.e. NiCd vs. NiMh vs. Li-ion).
now i'm gettin' onto thinner ice here as i'm having trouble recalling the following fact, i.e. whether i'm thinkin' of NiCd or NiMh. so, having said this, it's either NiCd or NiMh which loses ~2% of its remaining charge each day. the other batt types prob. lose something similar. that's NOT 2% per day, but 2% of it's remaining charge.
i've used a "ton" of NiCd, NiMh, & Li-ion rechargeable batts. other than the Li-ion batts in my laptop, i've become very disenchanted with rechargeables, having had so many go bad or perform poorly in my applications.
another other key pt. to remember about rechargeables (NiCd & Li-ion based upon my 1st-hand experience & perhaps also NiMh though i haven't myself seen this happen) is that if they are allowed to "fully" discharge NOT under load/use, they may suffer irrepairable "pole-reversal". i have never been successful getting pole reversed rechargeables (both NiCd & Li-ion) to recharge if this has happened. always long-term store rechargeable batts in a discharged state, by discharging them under load/use.
i'm not going to get into the concept of rechargeable batts so-called "memory". suffice it to say, there are actually two types of "memory" assoc. w/these batts (i've actually seen these referred to as "long-term" & "short-term" memory) and there is much confusion about them (even a bit in my own mind having read a lot of conflicting info on this subject - even fr/batt mfrs. i should have bookmarked it, but didn't - do a web search & hopefully you'll find that web site with info by a long time Battery Engineer - great info - he clears up a lot of myths about rechargeables and gives a lot of facts).
however, while i have had rechargeables go bad by deeply discharging them under load (at least the batts wouldn't take a charge again), far more often i have had them go bad by storing them fully charged & ready to go, only to NOT use them for an extended period of time & then when i finally used them they were "dead" - when they wouldn't take a charge, i would check them with my DVM (digital volt meter) & they would be in a pole reversed state.
oh...just remembered...i have had several sets of NiMh batts (in electric razors - get too many as gifts & can't use them all) go bad after being stored fully charged & then trying to use them 1-2yrs later. they wouldn't take a charge & upon examination they had suffered pole reversal.