There is currently no way to find an ammo box that has been put down by an engineer short of accidentally stumbling upon it.
It would be nice to get some sort of small icon on the map to find ammo, as I tend to place ammo all around for teammates but they will never find it…
Afaik there has been added some minimap icons for that in todays update 
I would suggest checking your settings and see if its turned off or on first.
Hahaha I just saw that today and I was so happy. I play engineer quite often and would slap the ammo crate in the opening leading towards the enemy cap so people at least saw it.
Cool, haven’t played since yesterday. Just happen to remember to get to the forum and say something.
Closed.