I wouldn't jump to the head gasket theory that quickly.
There are plenty of places to lose coolant without seeing much evidence.

Of our 5 bricks, one did eventually need a head gasket. But that's only one in five, representing a total of something like 650K miles* driven on cars purchased with 100-200K miles on their clocks.

Test:
Coolant reservoir has to be full within the level indicator marks. After driving car till fully warmed up, loosen the coolant reservoir cap. If system is holding pressure you should hear a definite "fffft" release of pressure. If you hear no pressure release then it's bleeding off somewhere. A sound system will still usually still have a pressure release even 30-60 minutes after shutting down a fully warmed up engine.

"hidden" leaks:
1) Hoses to cabin heater. Located on intake side of car, on front of firewall. Not sure how the left and right side stuff is laid out in the UK countries, you guys have your left and right all backwards. Anyway, it should be on left side of firewall, kind of low down where it's very difficult to reach the hoses for replacement. They flex when engine rocks on the mounts from acceleration. So will leak when flexing spreads the split in hose. If leaking there you should be able to see the gross slimy mess.

2) Water pump. Fan belt pulley covers the front end of shaft so you often see little or no evidence of the leak. The rushing air turns it into a mist spray and it's gone. With engine off, try put hand behind pulley shell and feel for wetness. There's (usually?) a vent or bleed opening at top of pump under that pulley shell.

3) Rear of radiator
It's hard enough to see the radiator with the fan and shroud in the way. Moving air may blow away the coolant residue. What's even tougher is a leak at very top row of radiator tubing. Had that happen once and we could not see anything till we removed the shroud retainers and pulled shroud back. Had to top it off and run engine to see the leak. Soon as we shut it down the fluid level was not high enough to reach the leak, so could not be seen.

* Previously I'd reported approx 500K miles driven on our cars. However yesterday's discussion with the final driver of Sophie ('86 245) revealed that she went to Volvo Valhalla with 502K miles on the clock! Which gave me a revised total for all.
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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors, quad horns, tach, small clock. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.