Hearing the ISS below the horizon?

I think I heard something odd yesterday, and would like others' input, if you listen to the ISS approach on 145.800MHz, FM.

Whilst I might expect to hear the ISS as soon as it clears the horizon - or even very slightly earlier on occasion, due to refraction and orbital effects, I wouldn't expect to hear it several minutes beforehand.

I observed this on two passes yesterday, each time a very weak signal being heard as the ISS crossed around 30 degrees north - when it was still well below the local horizon. The signal flickered weakly for a while, and then disappeared until the station became visible.

I don't know if it goes some way to explain it, but that is the tropical area where tropospheric propagation remains strong at this time of year. 

Has anybody noticed this before?