Random Chime but no event

This morning at around 4am I was awoken by my Homebase chime. Only the one chime, but enough to wake me (I am a light sleeper). I went and investigated all access points and checked my phone, but it had no events recorded.

Anyone able to explain this, or have had similar issues?

For info, my setup is this: House has Homebase 2, 2 cameras and 3 entry sensors (currently). We also have an annex that has a Homebase 1, 3 cameras and 2 entry sensors.
Everything has been fine up to this point.