Yeah... a slightly disturbing problem seems to be happening on grids that grow larger, and OSGrid is being more and more affected by it as I had this problem too 3 or 4 times now with the same sim. Apparently, Opensim's current grid system frees a certain slot that a simulator was on when that sim crashes or was shut down. This allows anyone to take someone else's established place while their sim is down and put their own sim there. And I don't mean someone who does it on purpose to annoy you, but while your sim is disconnected the given slot appears empty to everyone on the map and therefore a person looking for a new slot can choose your own location without any of you knowing. You only notice when you attempt to put your sim back up and see your location was taken by somebody else who has no idea. And unless you find the owner on IRC or this forum there's no way to inform the person they took your slot so your only choice is to move until you find a way to get in touch with them.
This needs to be given attention as soon as possible. As me and others discussed on IRC a while ago, best would be a way to lock a position on the grid to a certain grid account, so the user can take that slot as his/hers if noone else "registered" it already and name the sim name or UUID of the only sim allowed to start up there. I spoke about this on IRC and also wrote a Mantis report
about it, which I hope will be viewed and taken care of soon. So yeah, hope this issue is noted and will also be worked on to prevent these grid conflicts when people choose new slots.