At times, geocaching can be "on the fly" or at unexpected times. With that, a person may not have a writing instrument with them. If you find a geocache and want to log it, it's always appropriate to take a picture of the cache in its specific finding spot. Offer to email the cache's owner with the picture and be very specific of ground zero. If it's on an iron fence, explain which rod or corner it's in. Multiple benches in the area? Tell them which is the correct one and under which bench leg. Log your findings online and state that you have emailed the owner with your find.