I wouldn't advise relying on the mobile app: Many hotels globally will not issue the mobile app key/fully complete the check-in until after they've seen the guest (often to check a passport or something like that which is required for local regulations), seems to be the case in the vast majority of hotels in Asia / Europe. It isn't something I'd want to rely on. If the hotel did want to check or dispute your saying you were present, they will clearly be able to see that you never went into the room from their logs - mobile key or not.

The answer is: cancel in advance and set diary reminders if required.