You can see gases rising in many settings, from the jettings of clams at the beach to bubbles of swamp gas rising from mucky pools. Seeps of natural gas are another possibility. Earthquakes may disturb wide areas and release gas bubbles. So may the steps of large animals, like the dinosaurs whose footprints are preserved at this same site.
Features like these are also explained as raindrop impressions, but their concentric structure is more complex than the mark a raindrop might leave. Hailstones also leave marks in soft sediment. They might be distinguished by a wider range in size than raindrop impressions and an absence of splash marks.