Probably losing it but if its only a small amount then it could be evaporating before it hits the bilge. First things Id check would be pressure cap seal then engine water pump weep hole underneath look for pink or blue stains depending on colour of coolant.
I Cooked one of my engines due to loosing water from a tiny crack in the exhaust manifold by the turbo. Like you I knew it was loosing a tiny amount of water every day.
I slowed down to pass another boat and as I powered up again the low water buzzer went off I checked the temp gauge and all was good so foolishly silenced the alarm and carried on thinking I've only got 2 miles to go, kept a close eye on the temp guage and it never went up but as I approached the mooring and backed off I got a huge cloud of steam out the back and I knew it was terminal. Turned off and moored on one engine, by the time I tied up the engine had hydrauliced and wouldn't turn.
The low level alarm goes off after very little water loss but it obviously doesnt take much water loss to cause a damaging overheat and the temp guage doesnt register an overheat from low water so you need to be very careful