Random alert sound, no notification, sound not in list of tones available

Help! Ever since I got my iPhone 13 mini a couple of months ago, my phone makes a random sound with no notification badge or banner, and the sound is not an available tone or ringtone for notifications. It goes off a couple of times per day at what seem to be random times, and I can’t figure out what it is. Before you tell me to check each notification setting, I’ve done that! I don’t have any set to sounds only, and again, the tone isn’t even available on my phone.