It can only be one of two things (if all the connectors are clean and tight)
1. Battery failing
2. Drain on battery even with the key in your pocket.

A cheap volt meter is all that is required to find Item #1
Item two is just a question of elimination. Circuit by circuit. Most failures are found in areas like Media players still running, Nav systems still running, door lights and switches turning on due to door wiring chaffing.
Here the quick questions is "Does that vehicle go into sleep mode?"

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2005 X5 4.8IS
The Blue ones are always FASTER....

Current Garage:
2005 X5 4.8is
2002 M5 TiSilver
2003 525iT
1998 528i
Former Garage Stable Highlights
2004 325XiT Sport
1973 De Tomaso Pantera, L Model
1970 Dodge Challenger T/A 4 sp Alpine White
1970 Dodge Challenger T/A 4 sp GoManGo Green
1971 Dart Sport, “Dart Light” package
1969 Road Runner 383
1968 Ply Barracuda 340S FB Sea-foam Green