It is not abnormal to drain battery on an iPad that is turned on and not used for a number of days. If the iPad mini is old, it could be a sign that battery capacity os dropping.
As for the Siri 100% - the way that report works is it shows the app that is using the most battery over the last 24 hours. Since it had been well more than 24 hours since you last actively used the iPad, something had to be using the battery, and it was Siri - which is not just Siri, but the search facility on iOS, so iOS will periodically refresh the search indexes over time. It would not be abnormal for Siri and Search to show up on a report of battery if the device hadn't been used in her 24 hours, for sure.