If its small (a few inches maybe) and based on the shape and amount of grime on it Id say a lower case cover bolt. Could be from either side but Id check the chain cover first then move around the bike from there. A similar bolt is used at the foot controls as well (shifter side).

SOLD: 07 Black BA, 39mm FCRs, TPUSA stage 1 head, TPUSA 813 cams, TPUSA 10.8:1 pistons, TTP #3 igniter, Specialty Spares Long Cannons, Tsukayu Hard Bags. 82HP/55tq NEW: 19 Goldwing Tour DCT