I used to have a warning ding (mostly on exit, but sometimes on entrance) that I couldn't figure out, and never caught the message on the screen because it went away so fast. I started to notice a trend, however. It seemed to be more prevalent when I was wearing certain shorts or pants.
Long story short, I *finally* figured it out. Sometimes, when getting in or out, the fob in my pocket would get pressed and held on the top. This tried to activate summon, but of course it couldn't with the door open, so it would beep at me and display a message that cleared too quickly to read, because the pressure sensor in the seat changing state would clear the message.