2001 Opel Astra 2.0 diesel - service light comes on on idle, goes out when driving

After the engine gets a bit warm (less than 80C), whenever I stop and the engine RPM go down to idling (or I put it in neutral while driving), service light comes on. When I start driving and accelerate to some 50-60 KPH and put the transmission in 4th gear, the service light goes out. The engine sounds good and I only notice a slight decrease in power while service light is on. When I turn off the engine and then back on again, the service light doesn’t come on, even after long period of idling.
I tried reading the error code by pressing break and gas pedals and switching the ignition on, but nothing appears there. The check engine light never comes on, it simply doesn’t work and it has been like that for years.
The only problem that I know of is a small coolant leak that started a few weeks ago and I still haven’t had the time to fix it. I loose about 100 ml of coolant per hour. Temperature stays normal at all times, just under 90C, but I never noticed the fan coming on, even when idling at stand still.
This issue with the service light started after the car underwent emissions test. I’m a careful driver and I don’t think that I ever pushed this engine further than 3500 RPM, but on that emissions test, the guy was running it at full RPM.

Any ideas? Is there a coolant pressure sensor that could get tripped because coolant pressure drops on idle? Could have something gone wrong during revving engine at maximum RPM (it blew enormous amounts of soot from the exhaust, although there was no black smoke in normal driving, not even at full throttle/low RPM)? Could exhaust system be an issue? I drive mostly on open roads, but rarely more than 100 KPH, I thought this was enough to have a clean DPF.