low coolant level with no visible leak (no warning light) 1998 318i

[Charlie318i]

After purchasing my 98' 318i (150,000km) I had an issue driving where a heater coil hose came off and the engine overheated. This was quickly fixed by reattaching the hose and afterwards I testing the motor (idle and revving) to make sure it wouldn't come off again or that the head had blown. Now the car drives fine but I've noticed the coolant level is constantly low even after topping up. There is no warning light and I can't find any visible leaks or condensation in the engine bay or under the car. The temperature gauge tends to stay right in the middle although will periodically swing to low and then return later seemingly at random. My exhaust does not seem to blow smoke or vapour either.
What could be the issue here? Could it be a damaged head even though the car drives fine?

[MIKYZZ4]

The cooling system must be properly bled.
Bleeding the system of air should be sufficient if no damage was done by overheating.

[Tiago320]

Any diferences after bleeding?