Didn't see Sparkplug upon my return to the dealership, but I did find out a couple of things. Indeed, Sparkplug just wandered in one day and the positive responses from the repair crew enticed her to stay. She's been there for a "couple" of years now. No one takes her home at night, but she's not "locked" within the floor office. The staff installed a pet door so she can exit as she pleases. (I didn't think to ask if there was a "cat hatch" to the garage istelf.) Regardless, she lounges within the office "most" of the time. She's also been "fixed" so that probably reduces her temptation to roam. (No doubt that also reduces the odds of any "randy" Toms vying for Sparkplug's "affections".)
A business cat makes sense. The mouse population gets reduced as Sparkplug earns her keep and companionship is shared. Sounds good to me.