Hi @Jonathanprentice and welcome to this customer forum.
Some of what happens to your solar PV and battery power depends on how the system is set up. Generally, this will be to supply the property first and then charge batteries second. The third phase is to send excess to the grid.
What can happen depending on your system, is that as power demand changes, the system can’t always keep in step and so you might see occasional draws or feeds into/from the grid. Power switching is still behind some of the demand frequency and as the whole system needs an immediate balance in place you might see what you describe.
One other thing to bear in mind is that the system might be programmed to top up the battery charge overnight if it is below a threshold.
Just for interest, what type of system and inverter control do you have?