At the second "Gori" base(after "Supply Network" mission), going onto patrol with your 3 man team will result in your gear pool ammo loss even with 0 expenditure. The type of ammo lost matches the default weapon they bring to the mission
In my experience, going out onto patrol and return to base immediately will result in loss of:
18 30rnd 5.56x45 magazine
11 20rnd 7.62x51 magazine
5 200rnd 6.5mm belt
3 RPG-42 HEAT rocket