Was trying to fortify Rust Mesa yesterday. Planted AT mines, set up a MANA turret, and swapped to Heavy, so I'd be ready when the TR came (I knew they were coming, as I'd retreated to Rust Mesa to escape the closest base, which the TR were attacking.) However, upon sitting in my MANA turret for a while, I went to go check on my AT mines, and they weren't there. I know for a fact no one had shot them, so I assume they despawned. In short, what makes a tank mine despawn?
They don't despawn, someone probably went over them with mineguard or sniped them. There is a rare bug though which makes mines fall through the ground sometimes though.
But they were in my LOS from the mana turret, just not close enough to be visible. I'd know if they exploded or were run over.
With engineer belt I am dropping 5 mines at the moment. Then switch to claymores and drop them. Then equipping flak armor 5 for my engineer and still getting kills from everything I laid.
They shouldn't despawn, unless you place more than you can carry, or you log out. I think, maybe if you move to another continent.
In my experience they stay until you place some more elsewhere. If you can carry 3 and you place 3, they will remain even if you switch class, or change your loadout. If you go a distance away and then return sometimes they will not be rendering but they are still there as you can tell from your friendly explosive HUD indicator.