3 beeps

Seatbelt warning possilby

Was it cold outside when you heard it? In many BMW models, there is a warning when the temperature is below 37 degrees Fahrenheit, because roads may be slick or icy. Many European cars have that.

Also, the clock has a memo button that, when set, will beep 1 minute before the hour.

^That is also true, I totally forgot about that feature. My X5 had that, it is part of the newer BMW radios under the "memo" section.

My money on temp