We develop a level set estimation algorithm for a novel low cost sensor network architecture, where sensors are mounted on agents moving without an explicit objective of sensing. A level set in a planar scalar field is the set of points with field values greater than or equal to a specified threshold. The distributed algorithm uses opportunistic information exchange to estimate level set boundaries locally at nodes selected using leader election. Such estimates are aggregated at the base station. Effectiveness of the proposed scheme is evaluated using simulations with data from both synthetic and measured fields. Random way point mobility model is used for node motion and accuracy and trade off of coverage with communication costs is studied.