I hooked up a command block with the following command to a clock:
/execute @e[type=Player] ~ ~ ~ /testfor @e[type=MinecartRideable,r=1]
and found that the comparator going off that block was always on, whether or not I was within 1 block of a minecart (when I was the chat was spammed with "found entity messages). I know it's not really a bug since comparators usually measure the success of the first command in the block, but won't /execute always be a success so long as it finds the entity in question? (For those of you wondering, yes I am trying to make Payload).