Because patents expire, it is unnecessary to search back further than twenty years. Yet, searches beyond 20 years are helpful. Old patents may contain useful ideas or claims to critical components of Brown's mousetrap. If so, BrownCo has little or no infringement risk. The worst possibility is that a current patent covers part of her design, If so, BrownCo needs permission to use it.
Deliberate infringements generate larger awards, but ignorance is no defense.
Injunctions, too, can mean legal fees, inventory loss and retooling costs. Between those extremes, some component of Brown's design might be both unpatented and patentable. This would be good for her to know before BrownCo goes into production. They do not expire. The longer they are used, the more valuable they may become.