When I had this problem a year ago on our 95 960 it was due to the original grey coolant reservoir cap cracking and leaking a little bit each heating/cooling cycle.

DEWFPO
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1998 S90 071,245 and 1995 964 154,100

A 95 960 needs a new radiator (due to embrittlement of the plastic side tanks) along with new hoses and heater water valve if so equipped. Don't risk an engine meltdown.

Since I am not familiar with what engine and how the coolant flows in the 900's, this is only a general place to look.

On the 240's, the coolant for the passenger compartment flows through two hoses. One crosses the rear of the engine, the other originates on the left side and rear of the block. This latter hose is "S" shaped to allow for engine movement without breaking...

I chased a silmilar low coolant leak for 3 months before finding it on the under side of this S hose.

The coolant was spraying onto the hot block and dripping onto the oil pan lip so it evaporated even before getting to the ground. I found it by accident, attempting to get at the flame trap from the rear side and had a few drops of coolant drip on my arm.

The slit on the underside was less than 1/4 inch long, and right next to the nipple of the block. If I would have felt the hoses all around, this one would have been wet at the leak point.

It's possible that coolant is leaking into the engine, as in the case of a blown head gasket. You may want to have the coolant system pressure tested in order to best determine where the leak exists.
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dnvolvo '89 765T 200K - '91 245 100K

One of the most common source of leaks that don't appear on the ground is around the top neck on the driver's side that connects to the thermostat. Look carefully around that neck and you may find evidence seepage. It's a vulnerability of these plastic tanks. It could be time to replace it.
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John Shatzer, '97 V90 @ 115K