Yes, we do spot a lot of the dratted things. Usually just in time for a sharp jink to the side, or if really late, a pull up! Crews refer to them as "whip aerials" because the average tall, thin uncharted obstacle in days gone by was something erected by the local CB nutter; the term has stuck now that masts are going up all over the place.
If crews have the presence of mind they can store the coordinates of the "surprise" mast in the aircraft nav kit with a simple button press, and pass those coordinates onto 1 AIDU to prod them to update the obstruction data. Aircraft DVOF data (like GPWS but for obstructions) gets updated with the positions, but there is often a couple of months delay between a mast's construction and its appearance in the DVOF. Similar for the maps.
At various times I have seen flying units run a system of "local warnings", which effectively generates NOTAMs for distribution on-base only. This might cover local bird roosting sites, but also used to include new obstructions out-and-about in the low flying areas. Not sure if any units still do this.
An anemometer mast erected to survey wind at a potential wind-farm site near Marham led to the farmer whose land it stood on getting ostracised from his local community, and eventually comitting suicide (although there was a lot of speculation about foul play) so these things are obviously emotive!