I was out driving last week and noticed a wisp of white 'smoke' come out from under the hood/bonnet when at a stop light. I've seen this before on other cars I had and knew it wasn't right. I parked the car as soon as I could find a spot. When I open the engine compartment, I saw that the coolant tank had a dribble down the engine side of it. It dripped just a little bit of coolant onto the hot engine. That is what produced the 'smoke'.
The other cars I saw this on had catastrophic engine failures because of coolant tank failures. Those failures were a sudden loss of coolant due to massive ruptures. One engine blew a head gasket and the other got hot enough to warp both the aluminum head and aluminum block. Both were not worth the effort to repair. I sold them for parts.
So, yeah, when I saw that wisp, I stopped as soon as I could park it. About 30 seconds in all.
Thankfully the tank didn't rupture. Just a pinhole leak.
New tank is now in hand and will be replaced ASAP so I can get back to driving the 'vert instead of the SUV.

 

A good reminder to check fluid levels frequently. I routinely do the oil, and need to do the coolant more often.