First, there is the possibility of a civilization dying and evidence being erased by tectonic plate recycling. Mars is approximately 4.6 billion years old. We know it has volcanic activity, and most consider it to have inactive tectonics now. Some scientists believe it could have had significant activity in the past, based on the 3 distinct regions of Mars. Earth's tectonic plates vary in their rates of recycling: the oldest non-oceanic tectonic plate is 3.6 billion years old, and shows no sign of recycling. Oceanic plates recycle on the average of every 180 million years though.

Second, there is the possibility of technology that didn't leave a visible trace. This is not supported by any evidence and is mere conjecture. If the society tore down non-organic structures and converted to using all biodegradable/organic materials then evidence of the society would be scarce, if existent at all, especially if bodies were cremated as many do today. Chemical analysis could detect this I suspect, or more importantly prove it wrong.

Last, if for some reason civilization evolved underground and stayed underground, then there wouldn't be any visible evidence above ground. This is also pure conjecture. Micro-robots that explore Mars' caves might find something, as might ground penetrating radar.

We're not really sure what kicked off Venus' runaway greenhouse, so it's possible that would still occur, though it does seem less likely. Without the runaway greenhouse, Venus' atmosphere would likely remain primarily nitrogen. Its atmosphere would still be significantly thicker than Earth's but by a factor of 5 instead of a factor of 100. With the additional atmosphere, it might remain quite temperate even at the greater distance.

Of course had Venus and Mars developed in eachother's places, Venus would have developed as a smaller world with a thinner atmosphere (generally more mars-like) and Mars would have been larger with a thicker atmosphere (generally more venus-like), so the whole point is somewhat moot.