I've noticed that sometimes when I do missions I end up having more blood packs than I started with... Is this a glitch, or is there a way to 'earn' blood packs by feeding on zombies?

And to clarify so people don't make dumb comments... I'm not talking about my blood going up after feeding!!! I'm talking about my actual blood count going up when I haven't bought any blood packs.

How is this happening? Is there a way I can MAKE this happen, or is it a glitch that shouldn't be happening at all?