Judging from your description, I believe it is the transmission fluid that is leaking. It is clear red (some mfgs, like Toyota, use red coolant but I believe Volvo uses green coolant fluid unless replaced by someone).
Transmission fluid is a strange fluid; it expands as it warms up after the car is being driven, so this may be causing more leaks than at idle, as idling the engine doesn't warm up transmission fluid much.
Check underneath the car, if you can do so safely (set the shifter in "P", apply parking brake fully, chock the wheels with hard objects, or have it hoisted at a shop for visual inspection) and try locating the leak. I assume the leak is either from the transmission case itself, or from the transmission fluid cooler line that connects to the radiator. If the problem is latter, it will be a cheap fix; simply replace one O-ring + one seal ring. You can get them at:
Call them to see if they carry the rings; they may or may not be the same as the oil cooler lines. Also, make sure that the transmission isn't overfilled; this may allow fluid to escape from the dipstick. But don't let it go low either. To check for transmission fluid level;
1. Drive your car for 15-20 minutes. This allows the fluid to reach normal operating temp.
2. Park your car with the engine running, set the parking brake fully and chock wheels.
3. With your foot firmly on the brake pedal, go through each "detent" of the shifter, starting from "P", and stay in each gear for 2-3 seconds. The sequence is: P-R-N-D-3-L-3-D-N-R-P.
4. Open the hood, pull out the transmission fluid dipstick, wipe clean with lint-free cloth (NEVER get even a tiny piece of foreign object fall into the dipstick tube).
5. Insert the dipstick, then pull it out again. If the fluid level is correct, it should be within the "HOT" mark orbetween 2 lines on the dipstick. There may be 2 different level marks on each side of the dipstick, and you want to use the "HOT" or "Warm" side, whichever shows higher level. When you do this, be careful not to burn yourself as there are some hot engine components around the dipstick.
If the fluid is low, add just enough fluid to bring the level up to "HOT" or between the 2 lines on dipstick. Add fluid through the dipstick tube; a funnel with extension hose is very helpful for this. There are a few different transmission fluids available, and the one specified for 850 is "Dexron-III/Mercon".A quart is about $2-$4 I believe. There are many brands, but as long as the label on the bottle says "Dexron-III/Mercon", you can use any name-brand's product.
I hope this helps.