Loosing antifreeze

Back when I put in the new rad. I also put in new thermostat. Checked it, bad. The kid (PO) had bypassed the heater as it core no doubt leaks, he used a rad. hose too big, so when it got up to temp, it would start leading there pretty good. Drove it down the road 5 mi , barely gets up to the 1/4 mark.
When I changed out rad. I know I filled it pretty well full. I did not bother with drilling a seep hole in thermostat. I will watch it close to see if it developes an on and off leak somewhere.