Looks like lizard scat. If it is inside the storage shed on shelves, most likely Mediterranean Gecko. Mostly noturnal so not seen during the day. I have them on my house here in Texas and they do not care where they drop their scat. I have also noticed that it sticks to walls much like your picture shows. Not able to tell size of scat from pic but, Med Geckos are only 4 inches long or so. Scat is about 3/8 inch.

Look for them at nite with a flashlight. They will be very light color.

Hope it helps.

ECTimaeus