Battery draining, Ghost cranking with no key

My wife's '04 Tahoe Z-71 has been slow to crank,
like the battery is low.  Other times it cranks fine.  
Yesterday it wouldn't start, only clicking noise. I
brought wife to work and came back about 6 hours
later to look at it.  When I pulled up, I noticed the tail
lights and head lights come on then go off.  It was
in the garage, no key in ignition, in park, on off
position, trying to crank.  Lights would come on,
engine would make clicking noise, then lights go
off. It repeated that every few seconds.  I stuck key
in ad tried to start it, only clicks. After I pulled key
out, it didn't "ghost crank" any more.
Battery is good, had it checked.
I'm at a loss here, any advice??