In cases like this I always check the log to determine the location on global map, like mentioned. Then there is usually need to comb the whole area, 64x64 local tiles (that's over 1.6 hectare, quite a lot if it's a forest).
The best course of action is to prevent it, by dropping everything not needed on the zoomed-out map after spotting the enemy but before stepping on their tile. This also prevents them from picking up your items. Of course, it's only possible if you spot them in advance.
Personally I also use another trick - I modified graphics for arrows, javelins and spears to have a box of magenta colour. It may look ugly if there is a lot of arrows in one place, but it helps finding items in forest. I'm losing no more than 20% of shot arrows thanks to this. But I also use it as a marker for dropped items (and tanning skins in a village, felled animals when fighting a pack of them etc.), by dropping a single arrow (or broken arrow) over the stack.