Backtracking to the lake house and a quick search led to me finding Mom's phone on the floor right up against the toilet in the downstairs bathroom. Two things learned. One Mom relies too much on the Find my iPhone app. Two never put her phone in airplane mode unless actually on an airplane.
Losing Mom's iPhone also led to me finding her work badge in her car. Too bad we hadn't found it earlier because then we could have picked up the legs for the dining table top we already have at the lake house.