Downtown Silver Spring has a bunch recently. I got three in the last two days. One by Fenwick beer and wine, by the library, and by Woodside Park.
https://pokehuntr.com/#38.996040249365315,-77.02797889709473
The above link might help with determining spawning. I realize that it's not 2012-2013 data, but if you focus on just 4G network (2012 is roughly when 4G came out) and choose all available networks you start to see the pattern of coverage.
In theory the pattern of coverage should tie into spawning hotspots as signal coverage appears to me to be related as noted with the "hyper" spot in Rockville. If one looks around Open Signal's map you will quickly note that many Malls should be spawn locations, as well pretty much major interstate highways and other major roads as the cell signals seem to follow those patterns in the County.
It is not until you get into DC do you see the cell coverage become so dense that pretty much it is all over the place creating a uniform blanket of potential spawning if this theory is correct.
MrBot20