You're thinking of these "RFID" (Radio Frequency ID) gizmos they use now in stores to tag merchandise, and to tag kids and pets. They absorb some RF, and reradiate it, I think at a different frequency, probably half or twice (resonance) for maximum efficiency.

Let's say you want to radiate a signal just a watt in strength; the landers have been equipped with small radios in that range to communicate with orbiters, if memory serves me. So, if you want to radiate a watt, you need to illuminate the transponder with at least a watt. Let's be generous and say that the transponder can use, say, a square meter of the lander's surface for a receiving antenna. Now, since you don't know where it is, you have to illuminate an area of, again say, a square kilometer. That means you have to beam a megawatt at the surface.

Quite a load on an orbiter's power supply if that's where your beam originates. Or quite a narrow and precisely-targeted beam if originated on Earth; and it'd have to be coordinated with either the orbiter to be looking in the right place to receive it, or coordinated with an extraordinarly sensitive radio telescope on Earth. Remember, too, that the radiating antenna on Mars will be omnidirectional, wasting most of its measly watt.

Probably not all that feasible in practice.