Thinking about how the robots will seek the heap, a possibility is to set a beacon, like a light, that would serve as navigation guide for them. Another option is that they also emit light and the accumulation of robots, like a heap of them, would be the strongest source of light, acting as an attracting beam.
Sometimes the sensation of life related to the movement of a machine may be connected with the regularity and speed of the movement. For example, a machine that moves slow is perceived more “alive” than one that moves fast, and an irregular movement, with changes of rhythm, is perceived more alive that a regular movement. That may be one of the reasons why robots like Keiko Takahashi’s Meter Crawlers made out of off-the-shelf steel tape measures move like the were alive: