Yeah, that would make me curious too... I checked fuse number 6 on my 95 C280... vacant too. Just double checking.

I would take off the fuse panel and look for the wires underneath fuse number 6... maybe that would be a lead.

Also, do you have an aftermarket alarm? Seems like there could be a correlation because of hissing sound you heard (possibly vacuum pump; operates central locking system), and an aftermarket alarm.

Keep us posted!

__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)