im losing water in my radiator including the overflow bottle"??" reservoir

i cant see any leaks, all caps and hoses secured. all fans are working. one a 90min drive to makati temp usually jumps to 95deg in traffic but gradually lowers back to the normal 90deg. but it has never reached 100deg. and it hasnt overheated yet (even with the replaced broken radiatior since it was detected while parked and not on the road).

toyota corolla gli, 1996 MT. using a 50/50 water/prestone mix
replaced radiator with a surplus one about 8mos ago.

could i have a blocked radiator, it being a surplus? were does the water go if its not leaking or even evaporating. no traces i can detect. a faulty temp gauge perhaps, reading at a minus 10deg?

i want to know possibilities and try some DIY, before going to a mechanic


thanks