When you bury a lot of spheres - I'm talking 30-50 close to each other in a dead end (or just in the passage, but that would be harder to remember), if you give them a day, the walls lining the area get completely filled with stuff. So, yeah, the chance of finding fossils/other spheres is greater around the place you bury your spheres.
P.S. A lot of new fossils become available once you get a National Dex, you might try it out later I guess?