I do not, ever, leave the engine running while working under the bonnet.
I well remember a young gentleman mechanic 'fly away' from the front of my car having touched something particularly replete with electricity!!
My R171 350 had a power steering reservoir buried/hidden under the engine cover on the front right hand side of the (cold! and probably the same when warm) engine. The checking procedure used to (and still does) give me the horrors at the thought of dropping the cap into the black hole below the reservoir. The R172 layout is the same. I can't remember the position on the R170 320.
When I first bought the 171, second hand, I went through all the fluid checks in my paranoid way and found the 'dip stick' (attached to the underside of the screw-on cap of the reservoir) was bone dry. In fact it was surprising that the power steering worked at all as I drove the car home for the first time. I filled the reservoir with the proper stuff and never had to top it up again over about 30,000 miles and five years.
So much for 'Full Service History'!! Caveat emptor...