These are in Agyrtidae, the Primitive Carrion Beetles. They call them Flat Brown Scavenger Beetles, but most of the ones I’ve found patrolling at UV lights have been shiny and black. They range in size from 5-13mm, have 9 punctured grooves on each elytron, the pronotum is distinctively domed in the middle and flat at the margins, and they have 5 segmented clubs on their antennae.
They are found in mesic forests from Alaska to California, and are most active as adults from October to June. They feed on carrion, garbage, and other decaying organic matter, and Eric R Eaton (Co-author of the ‘Kaufman Field Guide to Insects of North America) reports once finding a “… boatload under a dead coot…”.
Habitat- Mesic forests
Range- Alaska to California
Eats- Decaying organic matter
Flight Season- October to June