Diagnosis
This is a battery pack protection circuit fault. The tool is detecting a cell imbalance or a high-resistance connection inside the battery pack.
How to Fix It
Remove the battery from the tool. Press and hold the battery fuel gauge button for 10 seconds. This forces a full reset of the battery's management system. Reattach it to the tool. If the red flash is gone, charge it fully on the charger.
If That Didn't Work
The battery pack itself has a failed cell or a broken solder joint. You need a new 12V Max Bosch Core battery. The GBA12V30 model is the standard 3.0Ah pack for this drill.
Heads Up
Do not try to pry open and repair these battery packs. The lithium-ion cells are spot-welded and the protection circuit is potted. It's not a user-serviceable part.