2 times in a row now I have not gotten credit for spotting killer croc, both times at the same place, close to the omac area. Edit: Got it the 3rd time, same place.. not sure what was different this time.
you need to wait for the fact that you spotted him to trigger, if you just run up and run away you do not get credit.
In my experience, at least with Croc, you have to wait for him to disappear before the sighting counts. If someone stands on the manhole he walks to leave through, you can block him, and he just paces around forever. If they move and he vanishes, then you get the sighting. So be sure you stay around until the iconics have their say and leave.
I was standing there for a much longer time than he was present, I already knew about that. When I spot one and activate them I normally just tab out into windows and do something else for a minute.
Using the alt tab function may be part of the problem. If your Client is not fully active when the server is testing to see if you are in range it may record a negative result. A Negative result would explain not getting awarded the marker for the feat.
If that's the case, it has only ever happened for me when it comes to croc, never not gotten credit for any others.