240 93 volvo coolant empty after a gallon was added the week before.

There are 4 specific points that you should check for leaking on a 240. the first is to take the upper compartment or radio out of the dash and look athe the heater core behind it. They tend to fail when they are old. Then check the 3 hoses from the firewall. 2 enter the head and block, the 3rd goes to a pipe (check that also) that runs next to the block, into the water pump. then, check the pump around the pulley area. I've had one leak out the front without dripping out of the drain hole. The last point you need to check is the top of the water pump. The top seal against the cylinder head is a pressure fit, it is installed by prying up on the pump and jamming it tight while tightening the bolts. It's goofy, and they tend to leak. If they all check out, just re-seal the pump.

Check the water pump for leaking