When joining a game in this case my server if you enable the debug to see the biters expansion data maybe it's more then just that (my f5 key and settings) it lets you see all the biters without a radar/player nearby as long as you want including any generated chunks even if not discovered by the player. Once you turn off the expanded area / debug it then drops that ability till you relog and do it again.
Video of it happening:
https://www.youtube.com/watch?v=XSTqmz4h6GE
f5 debug settings:



