"Mirrors holding/reflecting the soul" is a pretty common trope honestly, it appears in tons and tons of works. Mirrors have always fascinated, and in fantastical works been everything from containers to gateways.
Also as a practical matter it's frankly not particularly hard to grasp in this situation that the only "ordinary" object she chose chose to bring into this demon magic forest for some ritual might have some real significance. But it's Oscar works for an agency that as well as policing seems to deal with occult threats, wouldn't even be surprising if basic training included "smash the suspicious mirror/statue/[other common magic tools] rookies!" amongst other pro-tips.