In Battlefield 3 what can slowly damage my tanks?

Sometimes my tank starts taking damage and I honestly cannot figure out what the heck causes it. Immediately you want to think its a Repair Tool, but it's not, because I literally get out of my tank and physically walk around it and nothing and nobody is there AND my tank keeps taking damage while I'm looking around. As well as inside the tank proximity scanning shows somebody near. The next thing I think of is a rotary wing aircraft shooting at me with it's cannon, which CAN happen but drains a tank's health much faster than what's happening to me.

This is pretty slow-- at about one or two percent per second-- and a Repair Tool is much faster, but still that's not possible, because it has to be right up against the vehicle, and the damage is different than a Repair Tool, much slower.

Sometimes this happens around enemy positions such as a flag that they are holding. Other times, I'm pretty distant from any flags when this happens. I can physically get out, look around, and even look for rotary wing or fixed wing aircraft shooting at me from a long distance. It's also not possible that a heavy caliber machine gun is doing this, because that's what I originally thought, but just today this happened and I absolutely know that nobody was shooting at me with a heavy MG, or anything else for that matter.

This truly puzzles me and I cannot figure it out. Like I said, I have literally gotten out of my vehicle a dozen times when this has started happening and searched around and can't find anybody or anything doing this, including a little EOD because this continues damaging my tank when I step out, and once its destroyed, nothing is around me.

The only remote defense is quickly moving away from the area, but that is never 100% and it usually kills me due to being damaged and slow at that point. Also this phenomenon seems to start when my tank's health is low, I was at 25% when it started happening today.

So what can be doing this slow damage to my tanks?