likely coincidence, unless they opened your hood, or bumped something from
underneath, rad or hose... did they tighten something they shouldn't have
IMPORTANT: which BMW?
what type of plastic did you find in the driveway? and do you have a
independant BMW mechanic to go to?
An alignment shop won't help you, and driving it is risky, you could damage
the head gasket if it gets too hot, then you are seriously in trouble....
find a proper place and go there, don't just drive around!
if it is leaking, you will be low on coolant, so it really should be filled
before you start the car again, then