Keyless parasitic draw is miniscule. If nothing else were connected, it would run for a year before any noticeable drain on the battery, or, the natural discharge rate of the battery exceeds that of the Keyless.
Just sayin'
Now I just had to install a Battery Tender brand solar charger in our Mini Cooper S -- don't know what's going on there, but sure enough it's no good after three weeks.
Come to think of it, most machines don't like to sit.
I remember being involved in flight testing for the F-16 back in the day. At one time, there were six to eight planes assigned to test. The F-16 had wet wings, and was prone to leak. As long as a given plane was flown frequently (couple times a week) it would keep flying. If it sat for a while, the leak would be enough to ground the plane. And then, sure as (all those sayings) they would fix the leak, but then two other things would be wrong. And in the time those were fixed, something else would fail the diagnostics. One plane actually was down for half a year with its series of issues.
Our high strung machines are better than most -- still, don't like to sit.