I'm in northeast Indiana, USA.
As far as 173.5875MHz, that freq is used at a lot of air bases, however, one of them is using it as a control channel for a whole P25 system, instead of using it as a simplex P25E channel. I'm trying to find people around the Michigan, Ohio, Indiana, Illinois area to give a listen to that freq to see if they can hear a control channel. That would narrow down where it's coming from.