3 times now, in the last 2 weeks, I've added coolant to the reservoir on my 2005 Toyota Camry (125k miles) and after a few minutes of driving, it is gone. The car runs fine. Has never overheated and the gauge is in the normal range. I haven't been able to find any visible leaks, but I need to look more. My mechanic can't get me in for 20 days.
Quite awhile ago, I did notice a puff of white smoke every time I first started the car. But other than that, I haven't seen any of the signs you're supposed to see with a coolant leak. Oil looks normal, not milky. No steam from engine compartment. No puddles under car.
What should I be looking for or inspect? Is it safe to drive (in terms of not destroying the engine)?