Hey there... Sac'to as well :-)

to the werkz.
i wouldn't say weak spot, really...
On BMW's they use a pad sensor for the airbag/s that are susceptible to moisture when you shampoo the carpet or get the floor wet; drive in the rain; or ... some drunk passenger spilling a drink on the floor ... (for one reason or another).
johnny bayonne is also right in his post. In the 1st place, it is bad mech practice to toy around with steering wheel ans airbags without first disconnecting battery and letting it sit at least 15mins before commencing work ;-)
Not a major issue and definitely correctible. We're actually in luck bcz we have the recycle yards in Rancho Cordova where replacements can be had for cheap ;-)
I'd say seeing airbag lights on cars you have seen is purely coincidental. And if it's ON and up for sale, it's probably because the seller doesn't want to assume the load for repairs(almost always $1000+ at the dealers... for whatever reason).
You've worked on MBZ before... then u know it could just be a loose wire ;-)