[walrus01]

okay, high Mars orbit. Quite a lot less delta-v requirement for some theoretical several thousand kilogram chunk of long-lifespan data storage, and easier to discover and retrieve, than achieving solar escape velocity (look at the delta-v budget for the new horizons probe vs. its total size and weight, for instance).

[zaarn]

I'd probably put a signaling system on it. Some large RTG using elements with a long half life and extremely redundant systems that send a weak signal towards earth, just over the background noise. Some regular signal ("DATA" as a morse code) that repeats from an unknown object orbiting mars?

I'd bet that any civilization would be flying there ASAP to see what it is about.

The crux is really making a simple radio transmitter that can survive that long but the parameters are very weak (ie, the frequency is allowed to shift so long as it remains somewhat in a region that can be received on earth's surface, it needs a very long last power source but doesn't need a lot of power, unidirectional transmission is fine as long as it arrives).