Has it ever been considered to have an "aging" on reports?
I recently drove from FL to NH and spent more time indicating that Police, accidents, hazards of all kinds weren't there anymore.
If the police pull someone over they generally aren't there more than 1/2 hour or so. Vehicles on the side of the road sometimes stop for just a few minutes but could be there 2-3 hours waiting for service.
Why not start a timer appropriate to the type of report and when it counts down to zero remove it. Whenever someone confirms the hazard, restart it.
I would think you have the data in your system to develop appropriate "lives" for the different hazards.
Vladislav Rybak commented
Making the police icon more and more transparent until it disappear if no new reports were received