Good point about the install quality, had to re-do 3 in the family that were all approved fitted, was pretty poor.
All alarms will draw power, no reason why cliffys would be any heavier than others, the voltage you quote after 2 weeks of non use sounds good to me.
A few older toads are now starting to have issues due to back up battery leakage, as until 2 years ago they were NI-CD, after some years these leak and spill corrosive contents over the circuit board, or if lucky just go duff.
It is now illegal to use NI-CD, they are NI-MH much better, easy to replace, about £15, the battery back up draws power 100% of the time, a sign of a duff one is usually reverting to base settings and forgetting the stored settings, then total failure to turn off when set off, refusal to arm/disarm.
The original cells were made by VINIC, but I'd look at replacing if much over 7-8 years with a new NI-MH pack.
tabetha