It is more likely your GPS placed you in the vicinity (regularly?) with another AD ID that regularly searches for, purchases, or visits dog centric locations. It's also entirely possible that the other AD ID's (your grandma) dog food schedule is predictable and you happen to be visiting within a time frame of dog food purchases.

My grandma never owned a cellphone, only an old landline. And she buys dogfood in the neighborhood mom & pop store.

Well, we know for a fact it wasn't your mic being recorded. Maybe you walked by WiFi networks where people purchase dog food.

Or maybe the mic IS being recorded. We don't know it for a fact until all phone software is open sourced.